How old for a mini

[Smokin 440]

I want to buy a mini for my nephew once he gets a little older(he's only 3 mos now) but i was wondering how long i should wait before i get a 50 or a 90 etc and what brands or features you would reccomend... Thanks

[SmokinEX]

my little cuz started out when he was around 4, he started with a 50 4wheeler and is now 10 and has a ds 90 , he was always a good drive for a lil guy as far as i can remember