I was about 4 or 5 at the time.
The most important thing is that it's a fun and happy experience. They're so proud when they take off on their own!
My son and daughter learned at the same time. He was 6, and she was 5. She wasn't about to wait!
My granddaughter was terrified, even with training wheels. She's just recently learned, and she's turning 8 in August.
I had one child learn at age 5 and another at age 26.
Well, hard to say.
One of the boys kept them on because the whole neighborhood decided that they could do more tricks with the bike balanced on one or the other training wheel.
I finally suggested we take them off because they were becoming dangerous---they weren't designed for that purpose.