Owu, hearty congrats. At 18, go for the sea and stick to it. When I was 18 I applied for a Hamble Cadetship and failed, I tried BUA and failed. I headed for the RAF Cranwell Cadetship & failed. They automatically considered me for Supp list but I failed that too. Reckoned the Navy 8 year deal was good and I fancied flying on & off carriers in my Bucaneers but the RN didn't like me either. There was a fab chance with a three year commission in the Army Air Corps & possibly flying fixed wing beavers (whatever the hell those are) but failed the first stage assessment at my local Army Recruitment Office.
I wound up as a Loader at LGW. BUT, paths opened up and following advice of my Peers like Cap EN Jennings,Cap Moss and many others I wound up with 22000 hours as an Airline Pilot. I often looked down, from 40 odd thousand feet at sea going stuff and thought.........."Aaaaaah, that's the life". I met others who were ex fish-heads who, presumably, at the same time, looked up at me & thought ; " Aaaaah, that's the life" ! Oh, I did, in between all my failures try for a P&O Cadetship but failed the first interview and the British Merchant Navy fell into decline anyway.
Look, go for it, Very fortunate & enjoy a truly great career. Aviation is currently going right down the pan. Awful, awful, awful.