He destroyed them at ages between 4-9, we don't know the exact age though, so we don't necessarily have to go with the lowest end.
I'll also not gonna be indexing his stats like "At least Wall level at ages 4, At least Wall level at ages 5....", I'll just index it like, "At least Wall level at ages 4-9 (Much superior to no name delinquents at this age), higher at ages 10-14 (scales much higher than Soichiro)".
The thing is that we know he's at least 8-9 year old when he beat up the goons, but we have no idea if he's below that age, it's just a possibility.
There's nothing suggesting the opposite though? There's no detail that will change the fact that Chonbo and Chome being stated as the strongest dudes Kazutora met in Juvie, except for a scene of Yotsuya dudes beating them.
There's nothing that will change that Kazutora said they're the strongest obviously, the problem is what he means, based on just that statment, it could mean more than one thing, it could mean out of the people that remained in the juvie with him, they're the strongest. Let me make a scenario, let's say there's X gang, there are 50 members, some of them died in an accident including the strongest one, and they're left with like 35 members, and after quite some time, someone, let's name him Y, who knew the gang really well took the strongest out of the remaining 35 gang members and introduced him to train in Z facility, the head of Z facility might ask Y who's he is, it's as, if not more plausible that he'd answer the head of Z facility by telling him that he's the strongest X gang member, he may not want to explain every single thing about it.
Kazutora doesn't have to say everything, it's redundant and it has no relevancy to the main story.
Or Kazutora could mean that, Chonbo and Chome are the strongest when they're together and not necessarily individually.
The thing is we don't have enough information to conclude things so it's better to wait for what will happen.
Working under someone doesn't = to being below them in terms of fighting prowess. Examples of this would be Shinichiro leading 1st gen Black Dragons even though he's stated to be a bad fighter and Kisaki leading Moebius (Osanai being a really good example of how muscles ≠ leadership in TR) and Valhalla even though he's comparable to Base Takemichi.
Shinichiro had the charisma and Kisaki had the intelligence. You can tell how Kazutora has charisma as well as he was able to influence Yotsuya Kaidan members to work for him and take down Toman. He also has a big reputation through being a founding Toman member and being close with Kensho, who's stated to be known in whole Yotsuya.
There are so many other interperations of him influencing Chonbo and Chome, other than the "He beat them up and got them to work under him" interperation.
I am well aware of that, but the thing is Kazutora isn't necessarily comparable to the two.
Shinichiro is followed by others way stronger than him because of his personality, he never gives up, that's why he's looked up by everyone.
In Kisaki's case, his plans and methods works, so he's where he is.
Kazutora got neither of the above-mentioned qualities, so by default if someone works under him they'll be weaker than him, just like how things are by default.