I don't know the exact driver for it but it was probably some combination of an an interest by middle aged USTA members for more league play (and the corresponding opportunity to make more money through league fees) and a response to complaints from older "under 50s" about having to play the young guns in league play, or perhaps even young players complaining about having to play the old guys. Regardless, depending on your section and district and how it was implemented, there have been those that love the new division and those that aren't so enamored with it.
A recent thread on Talk Tennis started in support of the 40 & over division elicited some good discussion about the merits of it and also if it was resulting in a 40 & over 4.5 no longer being the same as an 18 & over 4.5. Some say that an 18 & over player will generally be more athletic and have more stamina and will have an advantage over the same rated 40 & over player. Others say that the algorithm looks at scores and if the 40 & over players are indeed losing, their ratings will go down accordingly.
I fall in the latter camp, that a 4.5 is a 4.5 regardless of age, and if the older 4.5 can't hack it, they will ultimately get bumped down to 4.0. Note that this hypothesis requires that there be some cross-play between the 40 & over crowd and the younger players for these adjustments to be made, but there are a large number of the older players that continue to play in the 18 & over league so I believe this is the case.
Now, if I'm right, what about the perception that the over 40 players don't do well when they face the younger players in the 18 & over division?
My first response is that this perception may not be accurate and may be skewed by a handful of results that people focus on from playoffs at the local, district, and national level where some young player crushes and older player 1 & 1 and thus not representative of comparing the play of different ages. It is probably more an indication of the under self-rating and sandbagging that is more likely to take place in the 18 & over division and an indication that the young player should be rated higher.
But another important point is that when you compare two players that are at the same level, they could be at opposite ends of the half-point range and thus if the younger player is say a 4.45 and the older player is a 4.05, the result is expected to be a fairly easy win by the younger player, but both players can be correctly rated as 4.5s. In fact, given age does catch up with all of us, it is probably reasonable to hypothesize that the distribution of actual ratings for 4.5 rated players is a bit lower for the over 40 players than for the under 40 players.
To test this hypothesis, I took a look at the data I have to estimate Dynamic NTRP ratings to see if any trends stuck out.
First, I took a look at the average dynamic rating I calculated for all players that ended the 2013 year rated a 4.5 and found it to be 4.23. This makes sense, you'd expect the average to be somewhere around the middle of the range, and the fact that it is a little lower than the exact mid-point just means the distribution of ratings is a little biased towards the lower half, also something you'd expect at a fairly high level like 4.5.
Next, I changed the players I looked at to only those that played in the 40 & over division. The average dropped to 4.21. Two hundredths isn't a huge difference, but it is significant enough that I think it supports the hypothesis. Note also that the 4.23 above includes these 40 & overs, so if they were to be taken out, the average for the under 40s would be a bit higher and gap would be more than two hundredths.
So I would contend that a 40 & over 4.5 is not any less a 4.5 than a 22 year old 4.5, but they very well may have a slightly lower actual rating and that is the reason for the perception that the old guys of the same NTRP level can't keep up with the young guys. It is just the nature of a rating system where a given level has a range of 0.5 rating points that this will happen, particularly at a higher NTRP level.