I worked in financial services for a while and I think Adsense qualifies as a similar financial service, though it might be different since we were working in a sector directly related to trading stocks and brokerage.
To comply with US legal regulations my company had to: verify the legitimate identity of account holders via SSN or other means, confirm they are over 18, confirm they aren't on a specific list of financial fraudsters or entities blacklisted by the US government, etc. If we didn't do this properly for every single user we could have been liable for all kinds of nasty legal ramifications, including losing various legal operating licenses we worked very hard to obtain.
A kid claiming they are older than they are is relatively low on the scale of identity fraud, but it is still a form of direct identity fraud and would have direct legal consequences if we allowed it through. To open the account, they had to have put in a false statement about their age. Verification isn't instant, it takes time to process, so you can easily open an unverifiable account on many services including Adsense, but it will forever remain unverified if you cannot provide legitimate verification info. I am sympathetic that someone of that age might not understand the consequences of their actions, but they claimed to be someone they are not (specifically an older person). Once you do that, you can't go back and claim to be another individual. They can find an adult and have an adult open an account, which would have various legal ramifications for that adult individual who owns the account, but it would be a new account for a new individual just as with Google Adsense.
Also some of the comments about data and GDPR are a bit absurd. You cannot commit fraud, spam, or otherwise violate terms of service, then demand all your data be erased under "GDPR" so you can open a new account and do it over again without recourse. We had constant repeat violators trying to game our onboarding and sign back up after being banned or denied an account. Showing any form of "grace" around previously closed or banned accounts was taking a big risk. We were able to do so on occasion, but only because we operated at a small scale, so an admin could look deeply into individual circumstances and make exceptions. Google Adsense likely faces orders of magnitude higher levels of abuse, and it makes sense for them to not reopen accounts for any reason to minimize their risk exposure. Yes it is unfortunate for those who were banned for a "minor" reason but there isn't much else they can sensibly do.