No problem at all, and about to get better (fyi) based on Markus saying a terrible bug/mechanix may be fixed/changed in the next patch.
Important game mechanics...
1) There are two files with their own associated spreadsheets used for importing players. You have Master.csv which references the historical_database.odb and MiLBMaster.csv which references historical_minor_databse.odb. The former consists only of players who reached the Majors and has less than 20K players in it. The latter is a MUCH larger list of players who have stats in the Minors at some point or another This file has over 100K players in it. Most (but not all) players from the first database are in the Minor League database.
2) I make myself a third file that merges these two files into one. I do this so that can include players with no minor league experience. The standard historical file includes debut year for the year they made their debut in the Major Leagues. This year determines the age at which they generate when you use their standard player id. The debut year in the Minor League database is the year at which their first Minor League stats appear. If a player is pulled from there then their age is determined from that value instead. By combining the files I can force the import for MLB players to come from the Minor League database... though this will not be necessary shortly following the next patch if Markus is correct. This large file's first two fields are player id and debut year... so I can always copy and paste any group of players I have filtered out into a 2 field importable file of as many players as I desire.
3) The player import feature can import players via a 2 field CSV file where the first field is player id and the second year is their debut year. By randomizing the players into 100 groups I end up with 100 files I can import that consist of 1400+ real players each. I delete a draft class when it is generated, and replace it with the players in one of these files via the Player import tool. That tool can be found on the drop down menu of the Free Agents screen.
Right now, I am waiting till next patch to rebuild my draft files. If the patch fixes a previous issue, then each of these files will generate players at 18 or 19 when they are pulled into the draft pool
All you do to make this happen is change their debut year in the file to the year they turned 18.
I skipped a lot fo steps and probably made it sound more complicated than it is. The hardest part, honestly, is making files that are clean and as evenly spread out for talent as possible. Everything else is cake after you do it the first time.