you have asked the question that nobody knows the answer to
its a matter of opinion based on peoples own experiences.
i keep mine in freshwater, and they are doing very well. they arent to hard to feed, mine will take frozen or live foods, and sometimes go for flakes and other fishfood. when i quartined them, they were just them in a tank, and were easy to feed and look after, then i placed them into a community and were still easy to feed, as the learnt to look out for the pipet with frozen food in. there isnt any aggression from these guys.
so to end with, no i dont think keeping in freshwater will reduce there life span, to me, they are found in freshwater so are able to be kept in freshwater, they are a bit like mollies in a way.