I was 23 when DD was born (7 weeks shy of 24) and loads of people were concerned I was too young. Not because of my lifestyle or immaturity, this was solely based on the number 23.
Tis stupid. My MIL on the other hand had DH at 18 in the 80s, SIL1 and BIL1 in the 90s then SIL2 and BIL2 in the 00s. She therefore can state catagorically that having a DS at 40 was the hardest, but then she had already had 4 kids, isnt the fittest person on the planet, gets no help from her partner, helps her eldest DD often with her toddler and has two kids at home.
I think having your 1st at 40 is very different from what MIL went through!