In general, vertical craze lines are nothing to worry about. Although it is true that the teeth are slightly weaker in these areas, unless you experience some form of trauma or repeated parafunctional movements (such as teeth clenching/grinding, or excessive nail biting), there should be little concern for them to chip/fracture and break.
If the craze lines are deep enough, over the course of your life, we could also see a little bit of staining deposite between the cracks. This doesn't occur often and if/when it does, tend to respond quite nicely to teeth whitening solutions and no "invasive" treatment is necessary.
At this time, if you don't have a habit of biting your nails, or using your front teeth to tear things (like adhesive tapes, potato chip bags, etc...) or grinding your teeth at night, then I do not believe you need to be overly concerned. However, if you do grind your teeth at night, then I would recommend considering getting a custom nightguard made and to wear it nightly to help prevent/protect your teeth.
If you're wondering whether "bondings" can make your teeth stronger, my answer to that is no. Bondings can enhance and improve the look of teeth that are chipped or fractured, or moderately stained. But the material used is just a polymer/plastic and will likely need to be replaced somewhere between every 7-15 years. You note that your 18 y.o. If we attempted to place composite
bonding right now, you'll be looking at multiple replacements that could possibly harm your teeth in the long run.
BotXXXXX XXXXXne, if it's not visible and doesn't cause any pain
, then your front teeth should hold up fine. I'd say in my practice, about 30-40% of my patients have some form of stress fracture (craze lines) on their teeth. They are asymptomatic and no treatment is needed.
JulXXXXX XXXXX, DDS