Is 170 at 6ft a healthy weight for a 18 year old guy?

Yes: Hi. That's a body mass index of 23.0, which is normal.

Yes!: at 6 feet 10 inches and 170 pounds your BMI is 23.06. Depending on your build and overall fitness, this is a healthy Body mass index. 25 to 30 could indicate pre obesity, over 30 is obese. Under 20, we become concerned about underweight. All of that must be taken into consideration with the entire patient health assessment and besides the extremes should be kept in perspective.