at what age can kids wear adult bike helmets?
 

I will be meeting a couple kids I've never met before for a short ride and need to have helmets ready for them. I will not be able to get their head circumferences before meeting them and buying helmets for them. The kids are 139 and 152 cm tall, so probably about 11 to 13 years old. Are they likely to be old enough to wear average adult bike helmets (size 58 cm +)?

Probably so, since they are adjustable.

By that age, most of the kids can use a medium adult helmet. I've taught kids this age in a bike safety school program and we mostly use adult helmets, but have some smaller ones for the few that can't use the adult ones. By that age, most of the kids can use a medium adult helmet.

Hard to say for sure, of course, but Giro sizing for Adult SM/MD has quite a bit of adjustability. I think Kali is also another brand with pretty decent size range.

Thanks for the replies, all! It turned out the bigger kid was comfortable in an adult helmet, and the smaller liked a 54-56 size kids' helmet best.

If the unfortunate children have a melon like mine, they can wear an adult-sized helmet by age 4.