Hi Caroline
There is no age limit, although every prospective adopter has to go for a medical to make sure that they are reasonably fit and healthy enough to care for the children. However, I do believe that there is a general guideline (although not official) that most Social Services go by that there should be no more than 40 years between the age of the parents and the child. However, it does depend on individual circumstances and can vary. My DH and I are both 41 and will be 42 by the time we are hopefully approved.
Your best bet is to contact your local Social Services and see what their guidelines are.
Hope everything goes well.
Cindy