Obviously the specifics vary by jurisdiction, but usually contracts that are 'necessary' (e.g. grocery store purchases) or beneficial to the minor (e.g. an employment agreement) cannot be voided simply because someone is under 18.

The further you go away from this line, e.g. a mortgage, the more likely a court of law would void the contract. As with many things in law, the specifics (if it makes to trial) is case-by-case and "it depends"; with settlement being generally based on a party's estimated chances of succeeding/costs should it go to trial.

> If a child goes through the checkout at the grocery store with cash, can the parent march in and demand a refund because "he's underage so the contract is void"?

Depends on the jurisdiction, of course. But for example in German law, the contract is not void exactly because and only if it was about daily necessities of low value - the law does, in fact, care very literally and explicitly about those details. So it's completely unfit as an example to generalize, and the contract with AWS would in fact be void. Their problem if they don't verify users' identities and age sufficiently - and it's almost certainly a deliberate business decision not to do that in order to reduce friction. and occasionally write off an unenforceable bill as cost of doing business.