There is not specific study that only looked at this age group. What we know is that finasteride could be used at any age after patient's external genitalia is matured.
As men get older, their testosterone becomes lower and therefore, their DHT is often lower. Finasteride works by blocking DHT, so if the DHT is lower, there is less an effect by this drug if a man's testosterine is lower. In men in their 20s, testosterone is usually high therefore DHT is higher and the drug is more effective in blocking the genetic hair loss problems seen with DHT