Personally I prefer an age gap between 5 to 8 years as the man (hopefully) will be matured enough to handle the relationship and the women will be still in a suitable age to be pamper and receive the proper attention as deserve.

In Asia culture age is one of the consideration in a marriage, usually the wife (first wife as a reference) is either same age of a few younger compare to the husband. Now days age gap is no more a big issue and people are more open minded that an elderly female with a younger male is consider tolerable in many society.

With medical advancement and healthier life style, many do not look their age and it is hard to gauge a person age by their appearance and with modern education and equality women no long subject to external elements for marriage but more in to compatibility.

Age Gap different have to be consider as how old he is compare to you. My opinion as follow

Man (in most cases)
Below 30 yrs old – Mentally not matured yet and need a lot of excitement in life as still full of some so much ego and energy.
Below 40 yrs old – Have faced some failure in life and understand the power of money (status) and learn how to appreciate more.
Below 50 yrs old – The golden age, here is where power shift for men, men become the prime target for women…Hehehehe……you guys know what I mean.
Below 60 yrs old – Consider matured enough to enjoy small thing in life, understood and gathered enough experience to provide love and guidance.
Below 70 yrs old– Every day is a new beginning.

Woman (in most cases)
Below 20 yrs old – Emotional and full of life and wanted to soak in whatever available.
Below 30 yrs old – This is the years, where she knew what she can get with what she can offer….
Below 40 yrs old – The most confident age range or the most desperate age range depending on self esteem and status.
Below 50 yrs old – Fine wine…..She have gathered enough experience to understand men can not be change, either to tolerant or just leave.
Below 60 yrs old – Every day is a new beginning.

Women are usually in most cases more matured then men by 5 to 10 years, it also pending on background and education level.