Way back in 1990 when I joined the Army at 20 I weighed about 120 pounds – after OSUT (Basic and AIT combined) I weighed in at 145 with very little body fat.
Well fast forward 17 years ::) and I am now at 158 – however my running has never been consistent – once stretch was 6 months and one was 4 months. Other than that it has been 2 months here and there. Add on top of that medication that is prone to cause weight gain and I ballooned up to 170 pounds.
As you age beyond the age of 30 it will take more work and attention to your diet to maintain your weight. I just heard on the news that a study found that the average person's metabolism slows by 5% each decade after 30 years of age.