At what age do kids usually begin taking showers instead of baths?
How do you help a child go about making the transition when the time comes?
Infant: When my first son was still an infant, I would sometimes take him on my arm and shower with him. My wife would take him out and dry him while I showered myself. He loved the sensation of water falling on him.
Age 1: We have a tub/shower combination so whenever he was done with a bath in the tub (on his own, with one parent watching from the side), we could briefly shower him to clean off that last bit of soap.
Age 2-3: Since around age 2½ we let him shower by himself, but still supervised. The shower head is moved all the way down to minimum height, which is as far above his head as I like to have it above mine when I shower. He still loves that, and we can tell that he's managing fine so we are now (at age 3) no longer supervising him all the time but just occasionally, but we do keep the door open and stay within hearing range.
Age 4 and up: By this age both he and us parents will have built up enough experience to handle unsupervised showers - but still helping with starting and finishing.
Any time, really - no transition work was needed as they saw that we preferred showers so obviously they wanted the same thing.
We started ours from around 18 months as they really enjoyed the spray. What we would typically do at that age is just take them in with us - and they can learn to close their eyes and tip their head back to avoid bubbles in their eyes between then and around 2.
By 3 years old we would let them sit in the bath while we used the shower unit in there - we would put the plug in for a while so they could splash.
By the age of 5 we let our son shower by himself, and the girls from about 6 - the difference here was solely in hair length. Our son has hair about an inch long, whereas the girls had over 26 inch hair and they just couldn't successfully wash it themselves.
I agree fully with Rory's answer. But something else that was helpful is doing graduated supervision. So first it's them in the shower with you, then you sitting on the bathroom floor or somewhere in the bathroom while they shower, and then it's them in the master bathroom and you in the bedroom if you have that set up.
My little sister is twelve years younger than me so I did this with her and it was very helpful. That kid LOVED showers and to this day you have to tell her to get out or she'd be in there forever (she's 14 now). :)