DS was using the mouse at 18 months, but not surfing or really playing with games, puzzles, starfall... until between 2-3 years. DD is 3 now and likes starfall. DS is 5 and is now on to video games. Pre age 3 I didn't limit time, because most of our day activities were parent lead. Meaning I didn't really want them on the computer more than say 15-20 min a day, but it was usually less anyway. And if they hadn't seen a computer or tv in several days (we RARELY watch tv here, maybe 2 hours a month), but then a rainy day and they were playing 30-45 min, then OK. So if I felt it was too much, I'd say "now we are going out. Do you want to go to the small play ground, middle playground, or bike to the large playground?" and we did one of these.
Now DS is 5 and he really likes his games, and I don't want to deny him his relaxing time, but at the same time I don't think it is really useful, so we use a timer. 20 min a day. Some days it is 20 min exactly. Some days it is 0. And occasionally it a bit more. But I think now I have to use a timer, or it could turn into a 3 hour habit. On the plus, I think it gives them sight words, so ity is positive for reading. But on the other hand, it is, imo, even more positive to sit down on the sofa or in bed together with an actual, physical book and read together, and they still get the sight words, and a lot more.