My 5 year old successfully built a Lego Porsche set made for ages 18+ completely by himself. What can I do to use his gift in some way? What does this mean, should I do something focused on this area for him?
First, if you can, buy more LEGO sets, regardless of the recommended age. Don't worry about buying a set that's too complicated: challenge is good, and will help your child learn to deal with struggle and success.
Second, from my own personal experience with my nephew, and how his talents were encouraged, I would suggest you tie your child's LEGO projects in to all of the areas of learning that are age-appropriate.
I say this because my sister and her husband allowed their gifted son to only work in the area where he excelled, and by doing this, their son fell behind in developing other necessary skills. For example, my nephew learned to read very early, and because his parents allowed him to read most of the time, my nephew's fine motor skills were neglected and the dispairity between his reading skills and drawing skills was an embarassment to him once he started school.
One way you can ballance your child's learning is to give them a LEGO set to build, then ask them to tell you a story about the LEGO set (verbal skills, story telling, vocabulary), and draw pictures of the LEGO set (fine motor skills), and stage mulitple LEGO sets to create a scene (imagination).
You could also work with your child to create multiple scenes, take pictures of them, and put them together to create a book, either on screen, or print the pictures and staple pages together. Creating books is a wonderful activity for all children.
Third, if you feel your child is gifted, knowing that as early as possible in their childhood has benefits. Gifted children are often lopsided in their development, which can be very confusing to parents and teachers, since it's their emotional development that lags behind their intellectual development. Gifted children have special needs, and the sooner a gifted child is identified, the sooner parents and teachers can address their needs and create a healthy home/school environment for the gifted child.
This article goes into depth about the social-emotional needs of gifted children.
As someone who grew up on Lego sets, and eventually became an engineer, I have a few thoughts.
Who chose the set? You or your child? If your child shows interest in things that you as parents choose, that's good. But you should also find out if there are sets that the child is specifically interested in. I never asked my parents for Lego sets because they were expensive and I knew they couldn't afford to buy many. But there were some kits that I wanted and never got. That didn't deprive me of development, but I would have experimented with other ideas if I had them.
Unfortunately, Lego today requires far less creativity than it did 30-40 years ago. Many kits have custom pieces just for the set. It's harder to "free play" because there is a lower ratio of generic bricks. Even so, I would encourage you to give your child a broad range of kits, because learning is ultimately a Big Data exercise. The more diverse the data you feed to your child's brain, the richer it will develop.
Foster breadth. As others have noted, channelling your child into being the next Tiger Woods or Maria Sharapova will make him a one-dimensional human. He will be much happier if you expose him to a range of activities. Again, variety is the most potent brain food. Lego is great because it requires manual dexterity. But sports games (even table tennis) and other physical games like Rubik's Cube also help build hand-eye coordination. It might not seem like having good hand-eye coordination is useful to say, a software engineer, but it is useful to a musician.
Music lessons. Most folks who achieve high degrees of academic success also play a musical instrument. Not only is music another good skill to broaden one's horizons, it is also very emotionally satisfying. It's also very mathematical, if one is inclined in that direction. A budding genius will appreciate its rich layers if given the opportunity.
Encourage depth. While it is good to dabble in a lot of areas, it is also important for your little one to dive deeply into one or two areas that really interest him. Your goal is not push, but to encourage. Most importantly, the way he learns to view failure will impact the rest of his life. If he sees failure as something that is met with disapproval, then he will lower his risk-taking and stick with "safe" activities where he knows he can succeed (but also won't push his own boundaries to the possible failure point). If he tries to build a Lego model that he invents, but it doesn't turn out the way he hoped, you should identify things you like about it, things that surprise you, and the parts that you think can be improved. Always make it clear that failure is simply a learning opportunity, and that when you fail, you should learn from it and try again. If he learns to adopt Failure as his personal teacher, then he will learn much on his own, and what he learns in school will just be a supplement to his self-learning.
Learn to code. Introduce your child to Minecraft early, as it is a good gateway drug to programming. Roblox also facilitates programming in a kid-friendly way, but for older children. There are numerous small learn-to-program apps for tablets. Regardless of what field your child eventually chooses as a profession, an ability to write code will be a benefit, especially in 20 years. The analytical skills gained from this practice are helpful in just about every other technical and scientific field, and even non-technical ones.
Let the child lead. While I could have skipped a grade or two in school, I am glad that I did not. Dealing with other human beings was always far more difficult than building things. Being different often results in unfavorable treatment by peers. It is just as important for your child to develop strong peer relationships as it is to develop solo skills. Getting him involved in team activities like band, sports, gaming clubs, etc. are good ways to find like-minded children who will encourage and support. Team activities are also excellent places to learn and develop leadership, and also how to deal with leaders (both good and bad). If your child shows a strong interest in skipping a grade or going to a magnet school, then have that conversation with him as a peer (take his desires and concerns seriously). Otherwise, don't push him to be a superstar if he likes sitting in the background. He will be much happier if you let him find his own way, but pave the road right ahead of him.
Humility. If your child easily finds success with challenges that drag other kids down, it will be easy for him to get full of himself and become an intellectual bully. Of course, many children learn this behavior from their parents, also. Don't let him learn it from you. Praise your child for his hard work, and remind him that he got to where he is because of the actions he took, and not because he is intrinsically better than everyone else. At the same time, remind him that a certain portion of success boils down to luck. This is a pretty hard thing to do for most successful people. We are strongly biased to take credit for wins and place blame for losses. But much of what happens to children is due to forces far outside of their control. And that shapes their fates far more than most of us are comfortable admitting. Not every child will get expensive Lego sets to play with. He is lucky if he does. Always place his success in the context of the advantages he enjoys, and talk to him about why some of his peers find life so easy while others struggle. You can't know all of the answers to these big questions, but they are worth considering in order to make your child a worthy well-rounded adult.
If you feel the build was quite complicated and hard, you can try giving other complicated lego sets. This may be a sign of your child being able to easily build things according to instructions, especially if he built it in a short period of time. Maybe you could provide a picture of the set? Your child could be quite proficient in following "orders" and carrying them out quickly. Your child could be quite creative or imaginative to build that by himself!
You could try getting him tested, but thats is optional. You can try discussing this with a professional as what @anongoodnurse has mentioned.