Seiza ... ouch!
I can't say that seiza still doesn't hurt when my legs go to sleep after fifteen or twenty minutes, but then that is from lifetime of not doing it and having my muscles turn into stretched rubber bands? If you don't do seiza, it will take three or four years in your fortys to even touch your butt to your heels, as it did me. That is a lot of pain ... ouch!
The weird thing about doing seiza is that the more comfortable you are doing it the easier knee walking and deep knee bending becomes when doing techniques? If you practice standing and sitting, then getting up from odd falls becomes second nature. Although with age and 280 pounds to push up, I find I am using one hand more and more often to stand up?
So, even if you don't see the benefits of seiza, and you do it as being polite, or as part of your dojo's regimen, you will benefit from its basic premise.
Oh, don't do it if you have a bad injury? Your teacher will understand.
Eventually, as your legs strengthen from practice, and stretch in seiza, you won't have any problem in seiza ... especially if you don't wait till your muscles are tight as stretched rubber bands, over forty?
Maybe that is why they ask me if I eat trees? Go Figure?