Calories in vs calories out is true at any age.

In the 30s, life is starting to stack up. Career, kids, etc. These things easily dominate your schedule and can keep you from eating well, or hitting the gym or sleeping as much as is needed.

Also, years of beer can catch up to you and there's so many calories in that.

Also get your blood work checked. If your hormones aren't where they need to be, it may be harder to build and maintain muscle. Muscle burns more calories at rest than anything else so slipping muscle quantity can result in a snowball effect of weight gain and unhealthiness.

Beer.