If it is a branch that have riders at her standard and do the progressive tests then yes she will get lots out of it - again, a good instructor should make sure she does. Our branch has several girls older than 17. You can join PC up to the age of 21 and provided you do you can stay a member...
I have mine split into 3. One currently not in use at all and won't be until summer. Managing the other two, weeks on and off and will get them harrowed etc in Spring when the horses will move onto the part that was kept so the other two bits get a short change to recover.
I would have thought all you were doing was making a statement that someone to take as they will, I think you would be admitting liability if it said 'Young Horse likely to Spook' or something like that but I thnk all your tabbard is stating really is caution.
Rob, my brother provides this service in Scotland - he is a qualified HGV mechanic and he goes to shows and does trailer inspections whilst people are competing, he also does twice yearly trailer services and has a number of customers - the particularly like that he goes to them as he is mobile.