Theresa- I like it. I don't think of it as particularly old. My mom's name is Terri, short for Theresa. I think Theresa is softer and a little older than Teresa, but not old enough to be an old lady name.
I also think Lillian sounds young enough. I could see it on an old lady but also on a middle aged adult, young adult, or kid. It sounds a bit stuffy and formal compared to Lily but I like it well enough. I also like Lilith.
Vera is more of an old lady name to me. I like it (with 'vair' and not 'veer') but I can't see it on anyone below 50.
Vivian is another more timeless one to me. I think of it as mature but not old; I know a teenage Vivian and it doesn't seem weird to me. I like it. I associate Vivian with Lydia for some reason.