There are more 20 - 30 year old players on tour yes, but these stats are simply showing what the skill level is of each individual year of age. For example, the strokes gained off tee is the average of all 20 year olds, then 21 year olds. On the tails, I agree with your point that the numbers fall off so the data gets a little more choppy, but it's still all averages of each individual year.
Good point and possible. What I find most interesting is the shape of each graph is different despite being the same players during this time. For example, clearly driving and putting go to the young players, approach shots to the middle age, and around the green to the "older" guys on tour.