This is a UK forum. Very few people here will have done the USMLE, as the majority of us work / plan to work within the UK. Assuming you’re a UK student wanting to work in the US in the future - make sure you understand how difficult it (getting a job as an IMG in the US) is.
If you search the forum there are some previous threads from people who’ve attempted the USMLE, that should give you some information. As for age limit, no idea - never heard of age limits for exams - but surely this is easily google-able if you’re that committed to doing it.