To an extent some weight gain can be expected and is even normal as young adults mature. That being said however, significant weight gain of more than 5 to 10 pounds can be considered unhealthy. It is especially unhealthy for long term health. Beginning the process of gaining weight at a young age sets many people up to continue weight gain into their adult years. This starts you on the road to obesity which can lead to type 2 diabetes, high blood pressure, high cholesterol, and joint problems.