I believe that those lines are craze lines. "Craze lines are merely cracks in the enamel that do not extend into the dentin." They occur because of stress in your teeth, ie: grinding your teeth, biting your nails, and even routine use of your teeth. What you've said also matches up with craze lines in that you don't experience any pain. Those craze lines aren't nearly as bad as some other teeth. Craze lines rarely lead to tooth breakdown, so you almost definitely won't have to get them replaced. There are some studies that say that craze lines may be spots where cavities can form, but these potential problems can be easily detected by dentists.
Most craze lines, like the ones below, are much worse than yours.
Craze Lines: Hairline Cracks In Your Front Teeth