You're thinking about sex with a 14 year old. Even though it was you long ago who had that experience, You're still thinking about a young teenager who is underage in a sexual manner. That person would also be 30 now, But you're thinking about them when they're underage. That's wrong.
Some thoughts can't exactly be described as 'right' or 'wrong' because we have no control over what we think sometimes or how we feel. I don't think you should intentionally fantasize about a sexual experience at age 14, But if the thought just comes to your head then there's no blame. It's natural and normal that we'd think positively about our first (or any) sexual experience if it was a good experience. If you feel aroused then that's ok, How else are you supposed to feel?
My first sexual experience was pretty great and what you are describing now is basically trying to convict "thoughtcrime" which is pretty wrong IMO. If the sexual act was legal then theres nothing wrong thinking back on it later. I know this is a hot take but come on guys, Lets stop playing around here and loosen up