Cracks found in internal walls

I have recently started to redecorate, I have been in my house for 3 years.
As I have pulled the wallpaper off near my staircase I have found a large crack running down the wall. I have also checked with my next door neighbour and she has a crack in her first floor bedroom wall, I have then peeled the paper back on mine and it's revealed another crack. She said the one in her bedroom has not got any bigger in the last three years,

The house is a mid terraced property around 150 years old. I have included a photo of the crack downstairs, could this be subsidence?

Probably historic and nothing to worry about unless it shows recent signs of movement.?

Thanks, just realised the picture is the wrong way, the crack goes vertical,
It also runs into the upstairs bedroom. Its been wallpapered for the last three years and I've never noticed it before. Is it safe to replaster and monitor it?

If it has not split the old wall paper in 3 years then it probably hasn't moved any more & is, as foxhole says, historic. The only real way to tell would be to fit a glass tell tale across it & leave it undecorated for 6 months but I would not have thought that necessary, it is quite common in old terrace houses. Personally I would fill it & redecorate.