Five year old son successfully built a lego set meant for 6-12 year olds... Is this common?
Yes, it's quite common.
The age range on the boxed toy is not based on intelligence or aptitude. It is put there to comply with recommendations, most importantly whether the toy is safe for the child to use.
In the US, the agency responsible for determining recommended age ranges for toys is the Consumer Product Safety Commission. Most toy manufacturers use these guidelines.
With respect to Legos, the Commission states:
One should place primary emphasis or importance on the following characteristics when determining the age appropriateness of interlocking building materials:
•Motor Skills Required
•Number of Parts
•Size of Parts
Our kids started with Duplos and early Playmobil (the figures of which - earlier Playmobil, that is - our pediatric neurologist friend called "little trachea people" because of the choking hazard they represented.) While they liked Playmobil, they didn't much care for Duplos and quickly took to Legos well below the recommended age limit. So, that meant they had to be supervised initially when playing with these toys (which to me was quite entertaining and informative. I loved the conversations my kids had while playing with Legos as I sat quietly in the background.)
None of my kids build anything, none are engineers, etc., even though they insisted on keeping their Lego toys (as opposed to donating them) into their 20s. I guess that made them Lego nerds, but in our case at least, their early interest in Legos didn't translate into any particular cognitive bent or skill.
Age Determination Guidelines – Relating Children’s Ages to Toy Characteristics and Play Behaviour. PDF
Toy age guidelines – lessons from Lego and The Simpsons