As with most seemingly simple questions, the answers are rather more complex. There are really two questions here. 

The first, how long do couples keep swinging, admits to a range of answers. Some never try it, some try it once, some are off and on, some see it as a hobby, some see it as part of their identity as a couple. Longevity in the LS generally means one of the last three categories. If you have met one couple, you have met one couple, which explains the two questions--how did you start in the LS and (more often implied than posed) what keeps you here? 

The second, about 55+. Passion has no expiration date. The question becomes why is there attrition at 55+?  More often than not, it's for health reasons, or for loss of interest by one or the other partner. It's usually not because there is a problem with the relationship. Marriage specifically and relationships generally have an arc; there's plenty of literature on this. What we do not see in older couples long in the LS is a threat of "grey divorce"--they are generally happy with themselves, happy with each other, happy in their relationship.  We know plenty of couples who were in sexless or otherwise unhappy first (and even second) relationships who seem quite happy and content to be in the LS with their current (and often final) relationship. We'll add that older couples in the LS tend to look after each other and make sure that health and wellness are prioritized. What does tend to change with age is the character of LS behaviors. The excitement of finding the next couple wears thinner, and many older couples have people they know well, have travelled with or encounter in groups and so on. The reason is that in the seventh and eighth decades of life (60s and 70s) there is a natural tendency to simplify and declutter life. The awareness that at least 2/3 of life is past triggers a focus  on "what matters to us".  Usually it is not NSA sex but rather sustaining and deepening relationships. 

You are not "late to the party", but it does beg the question "what has happened that has catalyzed curiosity and interest now?"

I'm over 55 and my wife will be there in a couple of years.

We still go to 'normal' swing clubs.  They take all ages 😃

2 hours ago, Bill George said:

How long do couples keep swinging? Are there places for 55+ swingers?

Age is not a barrier to having an active role in the lifestyle. A friend of mine, sadly no longer living, for a couple of decades organized with her husband (and after his death by herself) sex parties that attracted people from their 30s up through their 60s and in some cases significantly older.
 

If you are breathing and reasonably healthy there’s no reason you shouldn’t be enjoying partnered sex into your 90s. (My brother-in-law walked in on my 85-year-old mom and her 92-year-old boyfriend engaged in missionary intercourse.) 

I’ve posted elsewhere on the board that about ten years ago, when I was in my mid-60s, at a party I played with a couple in their mid-80s. (They celebrated their marriage in the late 1940s by hosting an orgy on their wedding night.)

I’m in a longterm ENM marriage and regularly play with one couples in their late 50s / early 60s and another who are in their 40s. 

We started when we were 53.  Yeah, people swing well into their sixties, many even further.

How long do couples keep swinging? Are there places for 55+ swingers?