Mine was when I was about 15/16 I think.
I found some lube after sneaking around in my brothers draw looking for condoms (random experimenting I guess?) and found lube too. Used it for masturbation and though why not I'll slip a finger in my bum. At first I thought this is so wrong. So I stopped. Maybe 6 months or so later I tried it again as when I'm horny I do literally anything. It was a bit better used more lube and played around a bit more too. Still wasn't comfortable with it though, but back in those days I thought if you were to stick a finger or something in your bum it made you gay so I guess that's what put me off from trying it properly.
Ever since I was around 17/18 I tried it with more confidence and ended up getting a few fingers in, sometimes I enjoyed it sometimes I didn't. Definitely made me rock hard though.
Since being 19 I recently gained confidence in myself to buy sex toys. Bought a fleshlight and lube etc.. and I guess I got so excited about it I just bought loads of stuff spending over £100 on toys and essentials. All come in very handy! I bought the Lovehoney Booty Buddy Silicone Butt Plug which should arrive on Monday so I'm looking forward to trying that out, but I also bought
Penis Smoothy Anal Dildo Prober 6 Inch to use and was kind of scared to use it at first, but it went in so nicely with a good amount of lube and it felt amazing. I can't stop using it and it makes my orgasms so spectacular! Just got to learn how to milk now.
So I guess long story short, when I was 15/16 I slipped a finger in so that would be my first encounter. However, I would class my first proper encounter around 3 days ago.