According to the Mormons' International Genealogy Index, which is generally based on State and Church registries of births, marriages and deaths, Annie was born at Scranton on 5 July 1894. That would make her 17 in April 1912. And at the time of the 1901 Irish census she would have been 6, but only a few weeks short of her 7th birthday. After studying thousands of census returns I know that many parents didn't pay attention to the heading 'age at last birthday' and gave instead the age at nearest birthday for young children, which in Annie's case would have been 7. With the same sort of reasoning, Annie herself when buying her ticket could have declared her age as 18 when she was in fact 17 but almost 18. We know of course that she told the American Red Cross she was 17. In a 1912 article about the Mayo contingent in the Chicago Evening World Annie is listed as 'a girl of sixteen'. Most of the information in the article appears to have been supplied by Annie Kelly, so perhaps 'sixteen' was her estimate of her friend's age.
None of this, of course fits in with Annie's own recollection in later life of being 15 on board the Titanic or, according to her own family, of having been born in July 1897 which would have made her 14 (going on 15). I'm quite prepared to believe that Annie's recollection of her age at the time of a particular event long ago could be rather vague, but could the info on her death certificate be wrong? Most certainly yes. I have my own grandmother's birth certificate, which shows her to have been several years older than the age recorded on her death cert. In my experience, a birth certificate is the only reliable means of assessing a person's age at any future date. And in my opinion, the weight of evidence available so far is in favour of Annie being 17 when she boarded the Titanic. To be sure about this, Mick, you need to get a copy of her birth certificate, which shouldn't be difficult. I'd also contact Senan, who might have further information from Irish records. Certainly I'd look at the 1911 census.
Lester, please pass the Panadol!