Circular Time indicates that Trakenites age much like human beings.Erimin. Hmmm? There was just something bland about her, despite the silliness of when ever she and 5 would bump into royalty that she would declare, "Oh, but I'm a Pharo don't you know?" Though I just crossed the viewing bridge between episodes 19 and 20, and in the arc of infinity the Doctors off hand remark about it taking him a hundred years to get around to doing some general maintenance on the monitor can leave one wondering exactly how long Nyssa's life span might be?
And the novel Asylum states that Nyssa only travelled with the 5th Doctor for two years: but, as with the page in Goth Opera where someone tracks the TARDIS's most recent landings (and hence establishes that there are no untransmitted stories between Kinda and Snakedance), that's one of those little throwaways which tend to be regretted and forgotten...
Personally, I'd prefer to go for Trakens having a longer life span - a decade or two between Timeflight and Arc helps a bit with explaining the 150-year difference between the 5th and 6th Doctorss age, and could be supported by the references in Keeper of Traken to a Keeper ruling for a thousand years (though he probably had help from the Source).