Not too young. Unfortunately, there is almost no age past puberty that is too young for spider veins. They are primarily caused by heredity and hormones and you have essentially no control over either one. As dr. Nielsen said, there is help. Spider veins can be treated very effectively on the legs with sclerotherapy. Please don't waste your time or money with pills, supplements, salves, etc. They don't work.
Sclerotherapy. Sclerotherapy (injection of a medicine in the vein) is your best option. The medicine can be Sotradecol, Polidocanol or hypertonic saline. It works pretty well. Laser is good also. Treatments are simple and although it takes awhile to heal, the results are pretty good. Hope that helps.
Can happen. To anyone at any age.
Nobody too young. However, starting as a teenager is not advised . Spider veins do come back and after years and years of treatment the time will come when nothing will help. My advise the later you can start the better having in mind that during pregnancy one will promote having spider veins almost for sure.
Several options. You can try a supplement called butchers broom, or you can see someone such as a dermatologist or find a medispa who treats with spider veins with lasers or injections, or see a vascular surgeon for other options.