IMHO it's not about age... but the players physicality and usually that's when most players are older 14u and up. What is it that you feel these bands are going to do for players this age? Make them better baseball players, prevent injury ? I don't want to sound curt I just don't get it.
2018 started using them when he was 15.5 and really started using them as part of his overall arm program at 16.5
Bands are good to get your shoulder and arms loose when you have something to get loose... if your a 11 year bean pole you are just stressing the tendons, joints etc... you won't get anything from it likely to injure the arm, shoulder etc...
long toss program is a better idea.. and pitching is not about strength , it's about explosiveness. IF this is about pre-game and you are 9 or 10.... whew... run a few poles...throw with a buddy and throw a few pitches... I mean dang most pitchers this age, have no idea how to pitch anyway... so much more to do than bands to make them better
I mean to each his own, but I doubt you see much difference using bands at this age.