I don't really know that there is one as such, but some LA's like no more than a 40 year gap between parent and child, saying that though I am pretty sure that there are a few couple who have a slightly larger gap particularly with one partner.
Hope that helps a little, if you are beginning to think about adoption, contacting your Local Authority (LA) can be a good place to start or you can get lots of good information from Adoption UK and BAAF.
Hope this helps a little and all the best.
Would check with your LA, but ours didn't have an upper age limit.
On our prep course there was a couple in their early to mid fifties who have adopted two boys who are 3 and 4. We also have friends who have adopted a 17 month old child - she is in her fifties, and he is in his forties.
I am 40 and my DH is 43 - we have recently adopted a 5 month old child - our social worker said to us that we are young in adoption terms!