In SERC and I think in EGB youngsters have to be 4 before they can do a pleasure ride. I didn't bring my youngster out till he was 6 - depends how sensible they are!
Some people rush youngsters into endurance, forgetting that it is a pretty demanding discipline, and they're in trouble by age 9-10. Do take your time, and enjoy!
In our endurance group, horses have to be 4 years or over and then can only do ten 10 mile pleasure rides in their first season. If you are considering buying a pure bred Arab (I read your previous post), they are late to mature physically and the top endurance horses are not rushed which is why there are a lot of horses in their late teens and 20s still regularly competing in endurance. My own Arab was 9 years old before I started doing any serious competitive rides with him - he went on to be an advanced endurance horse and we clocked up many miles together. Your best bet is to go to www.endurancegb.co.uk there will be a local group in your area where you can get lots of info and help. The trick is not to rush into with a youngster.
I'd agree with the OPs... arabs don't reach maturity until approaching double figures - that's mentally and physically! - and are at their best in their teens. A 6 year old arab is equivalent to a 4 year old of a warm or cold blooded horse.