14U - only really looking at pitchers and get a line on players, no college coach wants or is interested in committing to a 14 year old player, way way more risk than they would like
16U - this is absolutely the sweet spot, and to be honest it is more of a tiered system, the bigger D1s are scooping up the best pitchers and talent at early 16U, your lower D1s are trying typically looking at the young 16s. For instance most D1 schools will be closing out there 2017 classes for sure next Summer and are getting the 2018 class in order this Fall and trying to close it out by next Fall for sure if not next Summer. The NAIA and D3 schools are not getting really serious until you are an old 16. And will look at 18U players.
18U - I would not expect D1 schools to be that active at this age group other than at the Gold level where the elite younger age players would be playing up. But if you are at 18U I would not expect a D1 school to come knocking.
Additionally if you want to get signed by a high D1 then you better have the measurable. By that I mean foot speed, overhand throwing speed, and size. And you better have good bat head speed and be able to hit.
I have been told by many many many D1 schools that they typically only watch the 1-4 hitters on any team if they are just randomly looking at players. The feeling is that if you are not one of the best hitters on your team at that age group then you are not going to be a great hitter in college.
Last Summer at 14U my wife (who coaches) was approached by many D1 school and all any of them asked was who are your best 3-4 hitters. Not who is the fastest. Who throws the hardest. Who is the most clutch. Or best fielder. All they ever asked was who could hit.
So my suggestion to everyone is spend as much time hitting as pitchers spending pitching and you should be in good shape. Of and start making you boding into a athletic machine. You better be able to crush it if you are not fit.
