My gym encounter with this guy has turned problematic

Hi guys I'm f20 and I've been going to this gym for a years now last year this guy (m17) approached me at the gym we had a small talk and then he asked me for my number and I refused because he was 17 and a minor I politely declined saying you are younger and a minor and after that day we would have small talk occasionally and he was very sweet towards me for an entire year until last Friday he asked me out again I refused again very politely stating the aforementioned reasons and his counter was 'I'm 18 now and legally an adult' to which I told him I'm not interested in younger guys and dude flipped just lost his shit and yelled at me for 'leading him on' told me I'm gonna regret this and went back to his friends The next day as soon as I entered the gym he was there with his friends and I overheard him saying how girls like me lead guys on and that I was gonna regret rejecting him when it's too late now I have a habit of confronting people and that's what I did but dude just wouldn't let it go and for the past week he is always working out around me while talking to his friends about me leading him on I thought it would stop but it's been a week and I feel suffocated and just so enraged I can't change my gym timing and I also don't want to leave this gym it's my comfort workout space now

All I wanna ask is why are som teenage guys like this? And how to tackle this? I've tried explaining things to him like an adult and how his behaviour is getting problematic and this is turning into harrassment but all in vain so what other approach can I take? Or how can I handle it from a teenagers perspective?