Spider veins form due to genetic reasons, and are usually not common in young people. The are mostly found on women, but can be found on men. Lasers are often used to remove spider veins. Doctors can also use a fine needle to cut through the veins with electricity. Other than that, I suppose the only other way to hide them would be to cover them up with concealer, foundation, or even a spray tan may help.
They also inject them with saline solution...tho, these days laser would be the way I'd go if I had them.