I notice quite a few hairline crack in my basement concrete floor. I've lived here for 14 years and no water has come up through the cracks. The townhouse was built in 1999 (so it is not a new pour). I don't know if the cracks have been getting worse or not; I just didn't notice them before, but maybe they were always there. Is this something I should get fixed (how)? Or is it possible that they won't get any worse and water is unlikely to ever come in, if it has not come in yet.
Concrete slabs crack. Period. For this reason, "control joints" are often cut or tooled into slabs where aesthetics are a concern. They encourage cracks to occur along straight lines where they create less of a visual blemish and can better be accommodated by expansion joints in rigid flooring.
In your case, it's not likely a concern.
Hairline cracks are not a problem. If they get to 1/4" at any point, then you probably need to have an engineer give it a look over.
However, not sure if radon gas is a problem in your area, but cracks in the basement will let radon gas seep in, so it would be worth your time and the money to purchase a radon gas detector. You could probably call your city/county inspection office to see if radon gas is problematic in your area.
Welcome to crap construction in the 21st century, where nobody gets basements, just footings, and those that do get basements get 4" slabs on sand often above the frost line.
Whether water comes in is a function of the water table in your area and whether you have proper drainage on your property.
There is nothing you can do except install a sump pump. Don't believe the "crack repair" con artists.
Oh, wait you could jack the house up, bed the foundation with compacted gravel, use real foundation walls and pour a proper 6" foundation. Just kidding.