It's (likely) not a strategy. Other explanations exist. You need to know more than just the age distribution of founders YC invests in. You also need to know the age distribution of founders who apply to YC. It's silly to assume that the age distribution of applicants is unlike the age distribution of those invested in if you don't have the data to make that claim.

If, for example, 20% of YC-invested founders are in their 30s and 20% of applicants are in their 30s, then in all likelihood age is a complete non-factor in their investment strategy. YC applicants are just disproportionately likely to be younger, due to the fact that older people are more likely to have financial dependents and commitments which make it far more difficult (if not entirely impossible) to do the 3-month-long YC experience.