Any city in NH is gonna be safer than most cities in the country. Just keep general sense about what to avoid and you'll 100% be fine.

That being said, Portsmouth is the gem of the state if you can afford it. Portsmouth is expensive for a reason. It's fantastic.

If you do end up in Portsmouth make sure to check out Prescott park, the Portsmouth brewery and the library. Beautiful park, great food and drinks and a fantastic library.

Tons of night life. Lots of college aged people, but also 30s (I'm 32 and I spend a lot of time in Rochester (where I live), Dover and Portsmouth.

Durham is for 20s. Hop and Grind is good tho.