By nomad you mean the entity that crawls out from the chancellor in sp mission 11 or their grown ships ? Because they aren't the same, the ships themselfes are piloted by Nomads as far as I know.
EDIT:
I think they could have a unlimited lifespan, as they could regenerate infinetely given the resources or food to do so. I mean they were artificaly engineered and made by an advanced race, this detail was surely taken care of by them. If the nomads were left behind to guard their creators domain, and if I was one of those creators I wouldn't want my guards to have a "best before" tag attached to them.
