So I watched the movie (How to Date Billy Walsh) last night on Amazon Prime (in the USA). Sebastian Croft and Tanner Buchanan are both good looking/hot/cute in the movie. I don't think Sebastian or Tanner's characters ages are ever explicitly stated in the movie, but they are set in at the very end of year two of sixth form in the UK, which would be like the end of senior year of high school in the United States, so they'd either have just recently turned 18 or be close to it.
I didn't personally care for the movie. The story was hackneyed and trite, with a slight twist that was only mildly interesting at best. The side characters basically all fall into the high school cliques/cliches, and I personally wasn't a fan of the main female, Charithra Chandran. Not saying she didn't do a good job; she just isn't for me.
And since this is LPSG, if you're here just for the sex/hotness, this is a whiff; there are few if any LGBTQ themes, and there is nothing past kissing in the movie - no shirtless or underwear, much less sex scenes.
My recommendation would be to skip unless you're a particularly big fan of one of the actors/actresses. Its saving grace for me was to see Sebastian Croft play a non-villainous/bad guy role, but I have no desire to ever rewatch it.
I hope this mini-review saves someone 100 minutes of his/her life!