Extremely rare for a WG to ask for your age in my experience.
Maybe in passing conversation it might come up.
When I'm green light and I have my number displayed I ask for an age in initial communication if that is by text or phone call, I send something like.....can I just check your over my lower age limit please

I ask because I have an over 28yo policy, if they have read my profile then they will know this and I often get guys saying I'm in my 40's or 50's etc and that's fine, I don't need a precise age and I don't have an upper limit.
If I text back and say I only see over 28yo can I check you are older than that then I know I will get the early 20's lying and saying they are 28 to pass my screening, I've had it happen a few times and it's obvious when they arrive, so I end up having to refuse the booking at that point which is much more tricky because I get the 'oh but I'm here now can't you bend your rules ' 'it's only once I'm only 4 years younger' 'ill pay extra'
The lower limit is there for my comfort not the client