Travelling by air is a fantastic experience, whether it's alone or with some one else. The only annoyances can come when you're late and have no clue which gate to go to really!
Im only 18 now, but I go flying quite a lot, obviously when going on holiday with family, and solo for daytrips if I have nothing else planned
If you're fifteen now, you can still travel without worrying. BA
as far as I know, only care if you're under under 16, so in your case it'll be fine and dandy
(and I believe most airlines are similar)
As for places to go, how about Paris? (Orly Airport preferably) as everything there is centralised/easy to find your way around...
Quite interesting place Paris and a nice young drinking age...hic!
(But DON'T bother with CDG
airport as everyone gets lost there including myself!)
You could plan a nice route from the airport to somewhere everyone will be able to give you directions back to and go inwards using the Metro for ease (did this myself at Christmas).
As for Berlin, I wouldn't bother, I went on a little daytrip last Tuesday and although I had planned everything you could possibly imagine, it was (I wont lie) a complete bore (My idea was to look around the museums and look into the history). Mindyou the -9 weather with 3 inches of snow didn't help much in my opinion either
Anyways hope this helped,
Flown On: A319/A320/A321/A332/A333/AT45/734/736/738/744/DH8D/T204/T154/IL62/T134/IL-18/An-24