How to handle hand-me-down phone for teen

[Pregnantabroad]

I'm looking for practical advice for how to set up an existing handset for a teen. Do you know any good websites with practical guidance?

iPhone or Android ?
Nearer 13 or 18?

13 and android

when my older 2 got their first phones i downloaded an app called family link, i can see what they are downloading, how long they are on their phones for and how long for each app, i can lock their phones, send out an alarm if they lost it and track where they are all from my phone xx