First of all, I want to stress I'm not calling you a bastard B1ueberry and never would. However I am intrigued by your comments.
I am 38. My DP is 29. Do you think his mum has or should have an issue with that? Do you think it matters? Is it worse this way round, than if he was 9 years older than me?
Personally I don't think it matters, and as far as he or I can tell, his Mum doesn't have a concern. We get on exceptionally well. He's incredibly kind, funny and mature. Tbh I've always been slightly immature, and am often told I look much younger than my years, and when people meet us they don't even realise there's an age gap, let alone a 9 year gap. He has benefitted from me being older and therefore more established, as I bought a house in London in my very early 20's, when he was still at school (no, I didn't know him then) so now we own a nice property between us with considerable equity. There are benefits for us as a couple in the age gap! Plus given women tend to live, on average, longer than men, we both hope the age gap will give us longer together.
I genuinely don't see any difference between us having a 9 year age gap and a 19 year age gap: we're two people in a mature relationship, not numbers on a piece of paper.
How and why does it matter? Surely if we're happy it's no-one else's concern? FWIW my parents love him and often say they've never seen me so happy, or better suited with anyone.