There was a lot of discussion about the relative ages of the four hobbits, so I used the genealogical tables and did some math. In 3018, the year Frodo left the Shire with the Ring, he was 50. Sam was 38. Merry was 36. And Pippin was 28, not quite "of age".
I doubt their birthdays were after September 23 - wouldn't someone have mentioned it?