- Newborn to ~6-9 months: not mobile, and not too demanding either (especially early on)

- 6-9 months to 3 years: mobile and constantly trying to hurt themselves

- 3-5 years: mobile, mostly safe, but not understanding what "no" or "I'm busy right now" or "I'm on the phone" or "I need privacy" means; also not napping anymore, which means you lose that daily couple hours of bliss

- >5 years: gets progressively easier as kids begin to get independent and (somewhat) reasonable

Of course, this timeline is different for every kid.

We have four kids ranging in age from 1-7 years old. I work from home, but rarely am I in charge of taking care of the kids (right now, for instance, my mom is watching them while my wife runs errands). On the occasions where I am solely responsible for them, I consider any work I get done to be a bonus.