I believe that Tim got fired in his teens, and the flashback is just from few years before the events of "The New Jerry".
Since it's shown that Jerry recruits spies at early ages, and it's believed that Tim was orphaned as a child. So it's Jerry could have recruited Tim when he lost his parents, and possibly also because he was a genius as a child, but without any parental guidance he started doing bad things and creating stuff that wasn't approved by WHOOP - so they had to fire him (20 years before the episode).
After that he continued to do criminal things - since his criminal record is longer than Clover's list of ex-boyfriends - and managed to never get caught for years. But eventually WHOOP caught him just a few years before that episode, which is the flashback - because we only see him getting arrested, not fired.
So it's possibly that he's in his mid-twenties or early thirties in the present.
I agree with the issue about his age: Scam's already believed to be in his early thirties but you're right about him potentially being in his twenties. The timeline never actually confirms his age and it's never set in stone just when someone is chosen to work at WOOHP.
Not true. In the amazing spies, the spies are 10-13 years old (Another show made by the same people). Age 10 is still a kid too and they keep their secrets just fine. -_- Additionally, child prodigies are vast more intellectually developed than NORMAL kids and know how to keep secrets/act like adults. Scam seems the type that he would have been one of those, being a scientific genius. Also if he was taken in by WOOHP, it’s possible they were also his guardian. If that’s the case, they would have been watching him all the time and he wouldn’t get the opportunity to tell anyone.