Who is eligible for PTE, and is there an age limit?

If you are under 18, a parent or guardian must give consent before you can book; Pearson explains how during booking (Pearson). The Test Taker Handbook calls this a Parental Consent Form and says that, once approved, it covers all your tests until you turn 18.

If you use a child passport that is still valid, the photo must match how you look now. Pearson will refuse entry if it does not.

Your myPTE name, date of birth, citizenship and gender must exactly match your ID. The ID must be the original physical document (no copies or digital versions), valid, issued no more than 10 years ago and in Roman characters. If you test outside your country of citizenship, only a passport is accepted for PTE Academic; within your own country an alternative ID may be allowed if it is on Pearson's exceptions list (ID policy). Without acceptable ID you cannot sit the test and you lose the fee.

Full detail, including the UK SELT and PTE Core document lists, is in PTE ID requirements. Booking steps are in how to book PTE.

Eligibility is less about you than about the test your goal needs. Pearson's guidance (PTE Academic FAQ, PTE Core FAQ):

There is no pass mark to qualify for. Each university or visa sets its own minimum (PTE pass mark). If you miss your target, Pearson lets you book again once you have your last score, even within the same month (Pearson); the limits are in how many times you can take PTE.

Pearson updates its test and policies from time to time. Check the linked Pearson page before you book. This guide is general information, not official PTE advice.