US Citizenship After 20 years of residence followed by abandonment & renewed PR statu

My mother had immigrated to the US in 1970. She was given Permanent Residence at that time and resided continuously till 1980. Thereafter, she resided continuously but had one or two visits abroad; each of which was less than 4 months till 1986. Hence, she had a continuous residence (as per requirements) of about 16 years.

IN 1986, she moved abroad with my father (who is a US citizen). She abandoned her Green Card and obtain 10 years multiple entry visas for occasional visits to the US during the period from 1986 till 2009.

I filed an immigration petition in 2008 which was approved in 2009. She was granted visas under IR5 category. She has immigrated back to the US on April 22, 2009.

She is now 64 years old. The question is would she be eligible to apply for naturalization in consideration of her previous continuous stay of 16 years and her current age of 64 and being married to a US husband. OR does she have to again reside a minimum of 5 years once again under this new petition?