I've commented on an earlier thread of the same topic.
Mums 14 years younger than my Dad. My maternal GPs were very very against my Dad - for very good reasons. Not only was he older than my Mum but he was a lazy, work dodger too. My GPs had to buy my parents house then gift it to them as no bank would touch my Dad. My dad then managed all of 12 years work before getting laid off and spending the rest of his working life imitating Andy Capp. But that thats not the point of this response.
People do not age in a linear fashion. As you get older, you find your health drops off at a point rather than aging gradually. Yes, a 50 YO might be in good health. 60 YO? 70 YO? 80 YO? Nah. Ye,s a 70 YO might outlive a 50 YO but in all probability they will not.
My mum was looking forward to retiring + travelling - visiting GKs and the like. She managed 5 years before my Dad kicked off and she's stuck at home, baby sitting him. Not only was he a PITA when he was younger, all his irritating traits have become much worse as he's aged. My Mum is living with someone who behaves like an 80 YO toddler - and its not due to any dementia! Rather than retiring, my mum has become a OAP's OAP carer!
The poster who said Only marry and older richer man is correct. Money and finances just do not work anymore when there is a big age difference.
The OP's mum is right to be concerned. I would go nuts if my DD shacked up with someone 10 years+ her age. The OP is only seeing positives. The OP's Mum is seeing practicalities.
A little known but important point is that older men (50+) do have fertility issues. People always concentrate on old Mums having Downes. But older male parents also have children with genetic defects - mental + learning disabilities are very common. Do you want your child to have an older Dad? At 15 the man will be 65. What are they going to do together - collect his pension? Seriously, think long + hard.