" I'm aiming to do a bit of travelling, get some life experience and live off my own back in the big bad world for a while. Am I right that this would be a good thing to mention at interviews?"
It sounds like a good thing to get involved in anyway. Especially the self-sufficiency aspect of it. I've met endless "Uni" bods whose idea of "Travelling" is to fly off to Goa or somewhere for a year, get wasted permanently and have Mummy and Daddy send them more cash when they run short.
I wish I'd had more applicants like you when I was interviewing candidates for vacancies in my previous career!
I'm a Civvie, but I would guess that in the military, as in my world, attitude and aptitude would be a more important consideration than your age or how long you've wanted to be a pilot. If "Childhood ambition" was the sole governing criteria I'd be an Astronaut by now!
Guy Gibson (If memory serves) was a Wing Commander at 23 years old.
... You do know who Guy Gibson was, yes? 