-->> You should join teritorial army and you are eligible for it
17-43 as a soldier, 17-33 as an officer.
You can enlist past 43 if you are a specialist or professionally qualified.
ex-regulars can enlist up to the age of 52.
Apart from that you need to complete a full Army Medical and be of a reasonable level of fitness but those are the general requirements of the TA, although for some specialist roles you may also need specific qualifications etc