People don't always age in a linear fashion. Some do. At 40, they look 10 years older than they did at 30 and 20 years older than they did at 20. Others - especially those who aren't always out in the elements - seem to defy time. For them, age is more of an exponential progression. At 40, they look more or less the same as they did at 20, maybe a grey hair or two, or a little less fullness around the face, but otherwise unchanged. But time catches up with them in their late 40s, and they may age decades in the space of a few years.
Leliana is a classic example of the latter: her face is leaner than it was, certainly. Alistair is an example of the former. He's in his early 40s and it shows.
I'd put Vivienne in her early 40s.