Youth players are allocated the day before they are due with you, so if they arrive Day 1 of the season they will add a year to their age with the overnight New Season update the night before.
Arrange for arrival on day 2 and it will then not be a problem (for the first season) just be aware that depending on what level of academy you have, the next youth player will arrive a set number of days after the first, so for instance 16 days after, which would mean your 2nd allocation will arrive on day 18 of the season and then your 3rd allocation on day 8 of the following season
HTH