Seeing just a picture is hard . I like to touch, feel can tell you a lot about fabric. But in my opinion (an you know what they say about those :-) ...... I would say 60's to about 80's . Simply from the colors and I recognize some of the patterns from when I first started. As far as not finishing it or worrying about mixing fabrics. I say don't . Unless it is museum quality it really doesn't matter all that much in the grand scheme of things. I have one I just finished that has fabric in it from the 1930's to reproductions from today. These kind of quilts were meant to be finished and used. I do have one that I am hunting fabrics from one period but it is a repair job and that is what she wants.

My guess is 50s or 60s

I have some of the small blue flower on white background fabric. It's about 5 years old.

Some of the fabrics are from the 80's bacause that's when I started quilting and I have a few scraps like those.

I don't think I'd worry about it. I would enjoy making up the quilt for your daughter and have fun with it. You would have to judge by how old the other items are in the box you bought. Looks like a fun project!

Your lqs should be able to help you date them & advise what to do to enlarge it.

Some of these have been around forever. It is so hard to tell if they are 10 years old or 40 years old. Do any of them feel like polyester because that would date some of them. Some of them look like reproduction 1930's prints but not original.
I would mix my newer fabrics with what you have there. I think it would work out fine.

Your fabrics are from the 70s. I know because I recognize a couple that I purchased in the 70s!

I have no idea how old they are, but I don't see a problem with adding some newer fabric.

1970's... If you could do a burn test to determine the fiber(s), you could be more sure. I'd be willing to bet they are poly/cotton blends from the '70's...