It's a mixed bag. I can only tell you of my personal experience. This past summer, my 2018 played up on an 18u team. For personal development, it was fantastic. They made a west coast swing that lasted a little over two weeks. He left on that trip a hard thrower. He came back a dominating pitcher. The team, other than him, was made up of 2015's who had already graduated and 2016's. Most of the teams he pitched against were rostered entirely of kids who were already committed or signed, mostly to D1's. he went from a guy throwing mid-80's down the middle to a kid who knew how to work the strike zone. It was an invaluable experience for him. Keep in mind, though, that we only allowed him to do it with the provision that my wife made the trip (was not letting him hang out I rented homes with 18yo's as a 15yo). he did garner a lot of attention.
The bad: There were several inquiries. Unfortunately, at the 18u level, it tended to be JuCo's, D2's and NAIA - all schools looking for upper classmen. None had interest in him when they found out he was 15. Too early for them to even think about. He became the ace of that staff, even as a 2018 and is invited back this year to play 18u. Instead, we will be playing for a team that will play 16u and 17u. That seems to be the better option for recruitment. If this team is playing 17u and not 18u and your kid will get significant playing time, then do it.