You will get a better answer from others but I don't think there are seniors and juniors but you become an associate at about 18 [ someone will know the correct age] and you can stay a member until 25 now I believe.
they do have junior and senior camps though but I think the age varies
Don't think it's official. it depends on the Comp organisers. In our PC it's 14, or sometimes 16. Sometimes they wait to see who has entered and split the entries accordingly, so if lots of tinies enter the age split may be 14 if hardly any older ones.
For 'Senior' camp it's 10 and over, other PC's differ.
For Senior Area A test camp it's 16 because the meals are taken in the local pub.
Membership is available to children until the end of The Pony Club Year or the Centre Membership Subscription Year, whichever is relevant, in which they become 21 years old. Associate Membership is available for persons aged over 21 until the end of the Pony Club Year, or the Centre Membership Subscription Year, whichever is relevant, in which they become 25 years old. Associate Members must previously have been either Branch or Centre Members.