As-Salaamu ‘alaikum wa Rahmatullahi wa Bararkatuhum,
Thank you for your question. You mother’s concerns about her age are not valid in Islam; Khadijah, may Allah be Please with her, was 15 years older than the Prophet (saw) and he loved her and she loved him as much as any two people can love each other!
“Real” love (that which makes for the success in a “good” marriage) is respectful. So, if an age-difference problem presents itself, it would get dealt with like any other problem that would present itself in a healthy marriage, with respect and deliberation and patience, etc, because people care about each other and want to support each other and want to get along and not argue and fight.
If by “she can follow Islam”, you mean that you think that she can follow Islam, I think you better ask her first before you make that assumption. If she says “yes” that she can follow Islam, find out if that means that she wants to be Muslim, i.e., take her Shahada or does that only mean that she is willing to follow your rules in your marriage to her. Then, if she is not sure if she wants to take her Shahada, teach her about Islam so that she can figure out if she wants to take her Shahada – do all this before marriage to protect against her motives to take her Shahada being only to marry you.
While it is lawful to marry a believing non-Muslim, it is not easy when it comes to the nuts and bolts of the actual marriage process, especially in the way you rear your children together, you need to be on the same page. Her becoming Muslim may go a long way towards your mother accepting her, InShaAllah, because then your mother would not have the argument that you says she now has about raising your kids as Muslims.
It may help your mother to teach her a little about Buddhism (if your prospective spouse does not take her Shahada) so that your mother can understand your perspective spouses worldview—Buddhism is a very good religion, short of the fact that modern day Buddhism does not believe in God but only believes in good behavior (from what I have studied about it—maybe I am wrong)—which, by the way, is a very important thing that you need to find out about (if you perspective spouse does not become Muslim) because, when a Muslim man marries a non-Muslim woman, she has to be a “believer”, so make sure she at least believes in Allah.
It is very important to try to make your mother feel comfortable about your marriage, but if you cannot, she does not have the right to deny you the right to marry who you please, unless she can prove that the person has a bad character—which, from what you said, she cannot prove any such thing about this woman. This is the hard part – disobeying your mother when she wants something that you don’t want. But, you have the right to branch out on your own in your own life and choose your own path! While this may be hard for her to deal with, it is none the less your human right and Islam.
I think it is super important to marry someone with whom you have a serious affection—that is what will keep the marriage going through hard times! InshaAllah.
Beyond this, May Allah Make it easy for you and Guide your footsteps and make it easy for your mother—and may she make it easy for you!
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