It's certainly not unusual to have thin veins. However, there are certain things that can cause your veins to appear thinner than usual!
Veins and Dehydration It's likely that your veins will start looking thinner than usual if you haven't been drinking enough water.
If you're going to get blood drawn, it's best to start preparing yourself around 24 hours beforehand - you can do this by making sure that you drink at least 1.5 litres of water, and by avoiding alcohol (which can really dehydrate you).
Veins and Smoking
Smoking can cause your blood to get thinner, and if your blood isn't as thick, your veins will probably look thinner as well.
Veins and Eating Disorders People suffering from eating disorders often have thin veins, as not eating enough can cause damage to your heart and other vital organs. If your heart isn't working as it should, then your veins and blood-pressure will also be affected.