MY son was almost 5. My daughter is 5 and a half and still uses traning wheels. The summer we took off my sons trainig wheels, he rode his bike alot...almost everyday. That is key imo. If you take them riding 4 or more days a week, they get comfy riding and can go faster..and faster. raise the wheels. you will see if they are balancing or not. My sons learing to ride was unplanned. His training wheel broke and I couldn't find another set to fit his bike. I took them off and ran next to him. He took off, just went! Good thing that is the summmer I took up running...lol. I tried this past summer with my daughter. She can do it but doesn't have the comfidence...she gets nervous, never fell off, i made sure of that. When runing with them..hold the bike at first..then let go...if they get nervous say..i am here and touch there back..not holding them, just so they know and think you are "helping".
Make sure the bike isn't to big. lots and lots of practice riding with training wheels..move the wheels up. ride for a week like that, then remove them. If they fall off...get them right back on! hold the bike for that day if you need to,to regain confidence. If they are confident and not scared...they can do it.
With my friends kids...5 is young. my sons friend was 7 and only tried because my son, 5 was doing it. his bike was to big. our kids are the same height so we traded bikes, he got if right away with my sons smaller bike. NOW my son refuses to ride his 2 wheeler at 7. we got him a new bike..to big(thought it would be a good idea to get the next size up so he would get a few more years out of it): bad idea.