"You're Only Young Once" is the second Andy Hardy film but it's the first one with the entire cast we're familiar with and enjoyed in subsequent films. The first film, "A Family Affair" starred Lionel Barrymore as Judge Hardy and Spring Byington as his wife. Apart from that the two kids and the aunt were the same in both films. According to IMDb, Barrymore's advanced arthritis restricted his acting too much though there's no indication by Byington didn't return as Mrs. Hardy.
Most of this film takes place during the Hardy family's vacation on Catalina Island in California. The Judge is eager to go in order to go sailfishing. As for Andy and his sister, Marian, they both end up having vacation romances--Andy with a spoiled rich girl who is a bad egg and Marian with an even worse choice. However, as usual, the Judge does NOT jump in and try to split up these budding romances but instead uses the usual Hardy finesse...proving once again what a swell father he was...one of the best in film history.
Like most of the films in the series, this one is quite enjoyable and, as usual, Andy a bit too big for his britches. Nice family entertainment and well worth your time.
By the way, this is the only film in the series where I recall that there was an epilogue. At the end, Lewis Stone (the Judge) talks to the audience to announce that there will be several subsequent films in the series.