It's not though. It's one year of trying at 35. 1 year of trying at 36. 1 year at 37. Look up the statistics for live birth rates at those ages and above.
It's not just whether you can conceive - it's also the heartache of whether you're likely to miscarry, have to TFMR or go through multiple rounds of IVF.
I think there's a defensive attitude around this because for many many people they don't meet suitable partners until 35 or older, and it is offensive to suggest they shouldn't TTC or that they have missed the boat. This is not the case. Most people are simply suggesting that where a person is able to try earlier than 35 they ought not to wait, based on fertility.
If a person was unable to try before 35, and is only able to try aged 40 (for example) then I'm sure most people would simply wish them the very best of luck. However, OP is currently in a position where she could try, if she wanted to. She's feeling broody and wondering if that's a signifier to start TTC, or whether to wait. Posters are saying, based on personal and scientific reasoning, that she ought not to wait if she's in a position to try, because the odds are better now.
That doesn't indicate that nobody can fall pregnant older, or that they shouldn't, or that they are less-than younger mothers. Simply that, by trying earlier, she may avoid potential difficulties and pain down the road.
The same, of course, is statistically true for a 18 year old pondering whether to wait until she is 24. However, the decline is much less significant between those ages, and so many people would likely say there is such little difference that she can easily wait with no concern, particularly when considering social and cultural factors.
Social and cultural factors become less of a sensible basis to make this decision on in your thirties.