Congrats on the baby, Mahdoumi!!!
I can't believe people are being that strange to you.
Asking about the baby is your grandbaby... well, as it maybe could be the case, I'd just do as Cathy suggested and gently correct them. The ones making other comments... well, they're just rude and if I were you, I'd just smile sweetly and tell them that yes, she was a huge big change - for the better, as you love having her
If it makes you feel any better, my mom was considered "terribly old" when she had me, at 32
(and I'm number 3). My MIL was 35 when she had her one and only baby, my DH. FIL was 40. We do not, in any way, feel that our parents are old.
We're both 29 now, and have no babies yet, nor do we have any on the way (a mixture of our situation and medical issues, but still).