I have found a bit more information, and I'm pretty sure that Jane Bowen is Jennie Bowen, my great grandmother. However, I'd like to run the 'evidence' past members, see if it seems reasonable.
Her death certificate says Jennie Newton was born April 30, 1864 in Nantyglo Wales. Says her mother was Martha Bowen also born in Nantyglo, no father's name listed. Informant was her husband. Her daughter in law (my grandmother) told me Jennie's maiden name was Bowen and Martha's maiden name was Jones.
Her marriage license confirms her maiden name was Bowen. It doesn't give any ages, simply that he was over 21 and she was over 18.
US census gives 1891 as immigration year; I find a Miss Jane Bowan (transc error, should be Jane Bowen) arriving 2 June 1891, with destination given as Ohio. She's 28 with a date of birth given as abt 1863. Youngstown Ohio city directory for 1891/2 has a Miss Jennie Bowen listed. Youngstown Ohio is where she and my great grandfather were married. There were also several Bowen families in the area, it's possible there was already a Jane there, and my great grandmother started using Jennie then.
Going back to Wales; GRO has no record of a Jennie Bowen born in Nantyglo.
1871 Wales Census has a Jane Bowen 9 yo born in Nantyglo, with a mother Martha also born in Nantyglo. I did get her birth record from the GRO. Jane was born 30 March 1862, her parents are James Bowen and Martha Bowen nee Jones. The birth however, was not registered until 24 April, and Martha was apparently illiterate, her signature is an x with "her mark of Martha Bowen" in the signature box for the informant. I suspect the dates were conflated in later years.
In the 1900 US census she gives her age as 37, born about 1863. In 1910 she's 48, born in 1862. After that she gets younger every census--in 1920 she says she's 54, not 58. That seems to be the time that she drops the "extra" 2 years, as in 1930 she should be 68, if she was 48 in 1910. Instead she says she's 66.
I think I've found my great grandmother Jane Bowen who became Jennie Bowen Newton, but does this seem reasonable?