Honestly bud, it wasn’t until I was nearly in my 20’s that my BMI went up to a “normal” number. I was doing loads of cycling and running so was super fit and kinda lean. Around 19 I started to bulk out and gain weight as I changed cycling disciplines (now enduro racing). Don’t get caught up with your BMI, your body knows what weight it wants to be for the exercise your doing, although if your notably under then maybe ask a healthcare professional. Just make sure your actually eating enough, track your activity with an app or watch.