I've carried out extensive research into my Irish convict ancestor, Eliza Nolan, who was transported to Van Diemen's Land for seven years in 1842. The records kept by the penal authorities had given Eliza's age as about 17 when she was transported, however, a petition from her mother had said she was 14 when convicted. The judge who heard her case was the Recorder of Dublin, Frederick Shaw. He responded to the mother's petition saying that "The memorial is incorrect in stating her to be but of 14 years of age. She looked twenty at the least". When Eliza married for a second time in Australia, she gave her age as 44 in 1872 which suggests that she was 14 when she was transported in 1842.
Thanks to the recent addition of Dublin Roman Catholic records to the Irish Genealogy website I found her baptism at St Mary Pro Cathedral in Dublin on 25 February 1828. Catholic children were required to be baptized within a few weeks of birth, so Eliza was probably born in January or February 1828, making her 14 at the time of her transportation.
Thank you, Irish Genealogy!