Me eldest is 12 now, in 7th grade and now 5 feet tall. We started Muni about a year ago, she rides on a 24x2.6 Kenda Kenetics. She still doesn’t like 170 cranks, thinks they are too long. She can really cruise with 140’s but discovered the difference in hill climbing ability when she was schooled by a smaller girl that was at least a year younger riding with with 170’s at NAUCC on the hill climb.
I think in Nathan Hoover’s Sedona pics Beau Hoover is riding a 20" trials tire. I’m sure Nathan has some good advice on kids and Muni.
Depending on your terrain and their riding ability I would think a 10/11 yr old could handle a 24 muni with 150’s.
There is also a non-trivial gender factor that comes in to play here. Not sure if Unibrier’s daughter is considered “tall for age” or not, but 12-ish seems to be around the time that girls start getting taller than boys. She rules on a 24", but she’s also an advanced rider. For a 12 year old just starting out, a 20" might be better. My son is 10, rides a Pashley 20", and is not yet able to stretch to a 24". He’s in the 75th percentile for his class on height. I’d say in general, the rule of thumb is they can step up to 24" sometime between 10 and 14, but that doesn’t mean they should. Err on the side of more 20"-ers…for kids that age, the 20" is more versatile. I saw some pretty tall 14 year olds at CA muni weekend riding 20" trials unis on MUNI terrain. I didn’t see any 10 year olds riding 24’s munis on trials terrain.