In my case I was thin sinewy muscle even though I lifted heavy and took weight gainer.....and as you said all of a sudden at 24-26 urs. Old (can’t remember exactly). I remember I was with my girlfriend at the time and I decided to weigh myself in the scale section at a club store....and was surprised I was 15 lbs. heavier than I had ever been. I immediately thought the scale must be wrong ....so I tried another one and it was 12 lbs. heavier than before and then the third scale and last one said 16 lbs. heavier. I was kind of in shock....and excited all at once. I kind of forgot about it and decided to weigh myself at a friends house who had a scale in the bathroom and I was another 12 lbs. heavier than even last time. All this weight at this stage was mostly muscle building and filling out my whole body....didn’t really have a belly still just was a bit thicker all over.
Fast forward to many years later.....yes at 30 yrs old you will definitely pack on more Lbs. Mostly because of lifestyle changes along with metabolism. The older you get as a Man you tend to develop a bigger and bigger gut just normally unless you try to control it.
Birth. Seriously, I was 30 pounds at my one year check up, and my memories of childhood are heavily flavored with comments encouraging me to eat moderately. Basically all it takes for me to gain is to take my foot off the brakes for a bit.