16 years in my opinion is too large a gap, if one is looking for a long term goal of remaining married for ever. Many psychological and physiological nuances are there behind such a relationship. If the woman is say 28 years old and looks young and attractive, her man will be 44 years old and he may look middle aged. If the wife is an extrovert and mingles easily with the opposite sex, the husband may turn envious and start mudslinging at his wife.
If the wife is, say 42 years old and still has interest in sex, and if her 58 year old husband fails to perform, she may get highly irritated and the husband may suffer emotionally.
If the wife is 60 and her 76 year old husband dies, she has a long widowhood to face in her live without a supportive companion.
My ideal age gap is 4-7 years.
This article of mine could be of interest to you: