My guess would be that she was in her twenties when she started the project, given how she appears to age. I'd guess she's in her thirties or possibly forties by the end of the series given how her facial structure changes.
I find your observation Interesting because I had the same impression and my husband hasn't followed the whole series but when I did ask him how old she might look, he said that she looked in her late 30s early 40s tops. He also pointed out that she already looked mature when she handed the Beretta to a 6 year old Kirika. That's why I had the question regarding her position as the Kind Mother because she seems to excert an awful lot of influence over the sisters and the top brass of the Soldats respect and fear her enough to do al they can to undermine her efforts without launching an all out challenge.
Regarding the war, the Soviet invasion to Afghanistan could also be an option and it does fit the theory of an older Altena. In fact, I saw pictures of uniforms of Soviet Spec Forces and they're pretty similar, suspenders and all down to the Kalashnikovs (of course) to the soldier in Altena's flashback.
Regarding the screenshot from the 4th episode, since the date was included on a scene that is displayed for mere seconds, it might just be an oversight from the animators and these type of inconsistencies aren't that unusual in animation, so IMHO, we might need more instances of dates to actually construct a timeline that could point to 2010.