I wouldn't be so concerned with athletic scholarships, because few get full scholarships. I don't know how many are allotted for DI swimming, but probably not a lot, and most scholarship kids likely get partial scholarships.
That being said, an athletic kid who is a year younger may well be at a disadvantage. And perhaps she'll try other sports, and perhaps being a year older, bigger and stronger would be a greater factor in those sports.
It is tough to tell what the future holds. I suspect that if my middle kid had not repeated 2nd grade (putting her in the correct grade for age when she switched schools - though we did it for maturity reasons), she would not be playing sports now. Small for age, slow to grow, not real athletic - likely would not be playing two varsity sports and travel now had she been a year ahead.
Middle kid will never play DI, but could play DIII. And here is where the sport could help her get into her college of choice. Perhaps she would like to go to a NESCAC school. NESCAC schools (and similar) need kids who can meet the academic standards of the school, and also play the sport. A kid with high SAT/ACT scores, good GPA with rigorous courses can have the coach give them a "tip" or "slot", or even just put in a good word with admissions. This is where I see more value in the sports (or if you are DI material, or close to it, this will help with the Ivies).
You will find that there are more swimmers (track/x-country, tennis players) with high SAT/ACT scores than team sport types with high scores, though that is a discussion for another day.
