(May contain spoiler, read at your own risk)
Ok, so I'm pretty sure he's not actually taking years off people and animals, it's just that ever since he started taking care of plants, he's never aged and now, what seems to be hundreds of years for other people seems less than a few seconds for him. In other words, his time simply passes much slower and his concept/perception of time is much different since he had lived such a long time. That's why in order to show that, the author put the infant aging so fast in the panels next to each other.
(In the raw, I don't think the dog died right after, so this is why I think this way..sorry if I'm wrong)
May contain spoiler
I forgot about the bird, lol. Ok, so my reasoning as to why I think the bird died right after Dam held it was because it died from "fright". Young and small animals tend to either die or play dead when are frightened upon instincts. In the manhwa, it said how all the animals in the forest had always been scared of Dam and tried to avoid him. This is why such a small bird obviously couldn't take that much and died. I guess since Golden retrievers are often perceived as social creatures who basically like almost everyone and everything, he wasn't that afraid of Dam, he was only afraid of Dam in the beginning and ran away. Because he wasn't afraid of Dam after the last chapter(Chapter 18 I think), he didn't die of shock or fright like the bird.
Does this make sense? Well, that's just my reasoning as to why I think so anyways
I don't think it is like that. Even Yullie said that the bird looked bigger after burying it, you can check it, there really is a difference. And Mishee is shocked by his growth in just one night, after the birdie died you can see how he even looks at himself reflected in the water, comparing what happened to him to what just happened to the bird.
Even the first kid who grew, when Dam was still living in the village, was pretty shocked, the same as all the people who were around.
But I don't think Dam is absorbing anybody's life, I think they just grow fast. The plants don't die, they really just grow, and Dam stopped growing before making any human grow (the ones that he could seem to be absorbing life from). And he has been living far from other people for a long time and he never aged.
I checked a bit of the raws, so this next WILL contain a bit of spoilers.
I can't read Korean, so I really didn't understand a thing, but at some point the kids seemed to look a bit older, so I guess some time had passed, and one of them seemed to find a book that talked about Dam (probably not specifically about him, but some legend or similar about someone like him, I guess), so I think we will get to see some kind of explanation about what he is.
May contain spoiler
Oh, well, I think it was that Dam has the ability to make living things at it's healthiest but the bird ended up dying anyways. You remember the time the other guy (I forgot his name) was suddenly sick so Dam helped him but in exchange, his body grew? And all the plants also became healthier after he started taking care of them.
It was Chapter 5 I think? Dam said that the person looked to be suffering and "took care of it". Also, I think the baby did die of old age but it was only within a few panels because the author and artist wanted to show that Dam's time passes differently since he has been alive for so long and hundreds of years to him, feel like a few seconds.
I thought that "suffering" was more similar to having a nightmare than to being hurt, so I thought he just tried to calm him.
SPOILER about things that haven't been translated yet
I think Dam will have a flashback about that moment, so we will probably find our someting else.
And about the baby that died of old age, he really says that after the baby died, the people from the village came looking for "the baby", so they were still expecting a baby, not a lot of time could have passed.
Really? But then it wouldn't really make sense because it showed "the baby" going to school and calling Dam "hyung", but according to the plot, people who grow due to Dam's power only grow physically, not mentally, and thus, it wouldn't make sense for him to be able to talk yet if he only grew physically...
Okay so hear me out, what if dam has actually been taking years off peoples and animals life and that’s why he has been living for hundreds of years, like how that baby in the beginning got old in a day and died and how it was the same wish the boy and the bird, and also how his mom, sister, and sisters daughter died of old age