1. The rookie label was added to newly drafted players to show that they were newly drafted. 18 year old players without the rookie sign likely came from a newly created team.
2. Technically 19 year old marked as rookies do lose out on a year of training. However 19 year old rookies have a higher skill ceiling than 18 year olds out of the draft (meaning some of them can have higher starting skills which an 18 year old player will never have right out of the draft).
In overall, the "rookie" label was implemented because there was a period of time in the new season where the draftees are out but the entire roster hasn't updated yet. Previously if you timed it right, you could put your 18 or 19 year old trainee on sale, and some gullible users might think they are draftees and bid on them, only to find out that they accidentally overpaid for an undertrained player who wasted a season's worth of training when the roster is fully updated. So with the "rookie" label, you will know whether or not the 18 year old on sale is an actual 18 year old from the draft or if the roster just hasn't been updated yet.