Unironically, this is the best way. From the ages between 15-18 the primary driver of attribute growth is training. In fact if you move your promising players to the first team and do not play them consistently, then in fact you are depriving on average about 40 training days per season.
Between the ages of 18-23 - Training plus matches are required to further drive attribute growth, so players who aren't quite breaking through at that age, it is worth sending them out on loan for regular game time.
After the age of 23 - attribute growth is minimal and really you can maintain attributes with training and minimal game time.
Source: How training and match experience affect player growth - Evidence Based Football Manager