If signed at 4* near the beginning of a season, a token youth can gain nearly two stars by the end of the season, simply because a 4* player gain progress faster than 5*.

200% progress in one match without getting the MOM is typical for a 18y token youth. Such player at 4* can gain 5~6sp a day on average from match and training. If you have 28 green pack budget per season you can keep him above 6* for four or five seasons.