Why not?
It worries me when travelers who are over 40 pigeon hole themselves into certain kinds and destinations of travel, sometimes just because they feel that that is what is appropriate. I am not saying as a 40+ year old traveler, you should not go on a Caribbean cruise, or a tour of London Museums, bit I also don’t think you should feel obliged that that is the type of travel you must experience.

Lunch break
If, on the other hand, you, as an over 40 year old, have a yearning to follow the South East Asian backpacker trail through Vietnam, then I think you should definitely do it. It is important to remember that the classic backpacker trails are classic for a good reasons. (Keep in mind, it was our generation and before that pioneered these travel adventures in the 60′s and 70′s!).
An important thing to remember when considering your travel options, is you don’t necessarily have to completely conform to the backpacker or Hippie lifestyle lust because you are thinking of visiting a backpacker orientated destination. I see no problem with a partial backpacker experience, where you extract the parts of the backpacker travel experience you want, and exchange the bits you don’t for something more suitable.
For example, a long term, unplanned journey though Central and South America would be a once in a lifetime experience for any traveler. If you are over 40 and going to embark on this type of trip, I think you can easily make a couple of substitutions, as per below, and still capture the essence of an overland adventure, without the things you just know will make it impractical.
Examples
1. Substitute backpacker accommodation in hostels for 1 or 2 star hotel rooms. If you are traveling as a couple, this will often be no more expensive, and definitely quieter and more peaceful! We are definitely going be more comfortable with our own room, and not needing to rely on communal activities.
2. Substitute Full Moon Parties and big nights of drunken partying, for a couple of drinks in a picturesque beach-side bar, after a great local meal from a street vendor ( if your game), or a local restaurant. What you will save here, you can spend on better digs!
3. Substitute Bungee Jumping, Abseiling and hang-gliding for less adrenaline pumped, genuine cultural experiences, like a walking or bicycle tour, or a guided hike.

Exploring Hoi An by bicycle
But once you take out some of the experiences that 20 year old backpackers are loving, and make a few substitutions to more age appropriate activities, you will find the rest of the experience can be very similar. Things like using local transport, walking and hiking, vegging out on a beach and people watching in a city cafe are all part of an enjoyable long term travel experience and should be appreciated. Some of the best experiences of travel come from being away from home for long enough to immerse yourself in a different culture, and to this end, the backpacking experience has the benefit of being able to stay away longer, by spending less.
So, in summary, I think the 40+ year old traveler should definitely get on the backpacker trail. It is travel, as against a holiday, and can be life changing, even at 40(+)! I am terming the phrase “OLDPACKERS” for us - Just like backpackers, but older, wiser and crankier if we don’t get enough sleep!