Welcome to Aikiweb!
There are many of us who started training over 40, so that certainly is no bar. In terms of physical limitations it really is a matter of the individual instructor and the dojo culture he promotes, not a matter of the art itself.
There are people training very well who are missing limbs, are unable to see, have spinal issues, cannot hear.... But any given instructor may not welcome such a student; in your case, you might hear "doing technique from kneeling is traditional and you should work on developing your ability to do it" while another might say "just kneel to bow in, you can do everything else from standing"....
Full disclosure: injuries are not uncommon in aikido although there is a lot of variance dojo to dojo (as I found doing my survey on knee injuries). A common beginners injury while learning to roll is a shoulder separation; ie, a disconnection between the far end of the collarbone from where it meets the shoulder blade. So going slowly and at your own pace and with good teaching is super important.
I suggest you visit all dojos in reasonable distance and watch class and talk with the instructors. Each of us our own criteria and gut feelings and it's never a matter of "one size fits all" - hopefully you'll find a place that feels right!