For pedal bikes, rules are light almost everywhere: rental shops rarely impose more than a minimum age for unaccompanied rental and a deposit, and no license is needed. App-based bike shares typically require users to be adults or mid-teens depending on the operator and city. E-scooters are stricter, with minimum ages commonly in the 16 to 18 range set by law or by the app, and a few countries now require at least a moped-class license or have banned shared scooters entirely.
Helmets split into law and practice. Some countries mandate them for all riders, some only for children or for e-scooter users, many not at all; fines exist where mandates do. Rental shops usually offer helmets free or cheaply, while share schemes mostly expect you to bring your own, which almost nobody does, so if the local law requires one, that is on you to solve.
The rental app itself is a decent first source since it displays the local age and helmet rules at sign-up, but confirm against the city or national transport site if you are unsure. And whatever the law says, a helmet on unfamiliar streets is simply cheap insurance.