Hi Shelly,
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.
Viva
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