In terms of keywording for stock (versus general discussions about how you categorize age), I think the Getty CV has the brackets in reasonably good shape and a Mature Adult (Mature Woman, Mature Man) is between 40 and 59. Senior is 60 and over. Young adult is 20s, and 30s is Mid Adult (I think the wording is awkward - no one uses that in speaking to people). I think at Veer (using Corbis' CV) if I keyword "older" it wants me to choose senior or mature adult for a meaning - i.e. they don't see mature as synonymous with senior either.

Perhaps as time goes by we'll have to divide senior up into categories 60-79 and 80 and up - all those ads for "active seniors" showing gray haired people running, climbing mountains, etc. are talking about a different senior from the more frail folks in their 80s and 90s who appear in ads for assisted living facilities.

Both "mature" and "middle-aged" are appropriate for someone in 40s or 50s. 

(this is not directed at the OP, who didn't ask about the word senior, but at those suggesting the term)  "Senior" really refers to people at retirement age, so over 60.   People spamming every model over 40 with the word "senior" is a big problem.  Nobody is looking for somebody 45 when they type in "senior". 

unless it's editorial, the actual age doesnt matter - if a 45 year old LOOKS senior, it's a good keyword, just as many twenty year olds continue to look like teens

There are some large grey (haha get it?) areas when it comes to defining a persons age by keyword but 45 is not a senior regardless of what they look like.