Is 131 lbs for a male (20 yo) underweight or average?

Depends on height!: Unless you know a person's height, a definite answer cannot be given. You can have your leanness assessed by a vascular impedance study: normal for a male is 17-23% body fat, but athletes can be much lower than this! also, not factored in is your frame or bone size. Smaller boned-lighter weight! good nutritional support is vital if you are underweight! and what sporting activities do you do?

How do you FEEL?: I was 6'0" and weighed 131 lb during my senior year of high school, when I was doing a lot of swimming. I'm naturally light-framed. I was lean, happy, healthy, and felt great. Ignore the "ideal weight" tables -- you're not a number. For a shorter guy, this may be "average weight", but the key is that if you feel well and are competitive in sports, your weight's probably fine.

Incomplete data: If you are the size of an average adult male (5'10")you are in the lowest 5% of weight for height at that age. If 5 foot, you would be close to the 75% H/Wt. At 6' you would be quite underweight at below 3% for H/Wt/age.When you post a question the more relevant data you provide, the more likely we can provide a specific answer.