<laughing> I started putting my son on timeout before he was 1. Cribs are WONDERFUL for this.
I would explain what was wrong, & what the consequence would be, and then scoop up (with an "expression", and not a happy one on my face), and deposit him in his crib. I'd leave the door open so he could see me going about my buisness. At this age time out was EITHER for a minute or two, or until he STARTED "calming down"...yep sometimes he went in his crib in full redface scream mode. When he would start calming down, I'd praise him for it.
Before getting him out of the crib I put a big smile on my face and would remind him why he was put there; "You went on timeout because you hit Susie Derkins. We DO NOT hit our friends. You can come out now, as long as you're playing nice." If he was mean he went right back in.
Same for the park. Even if we JUST got there and he (threw woodchips, hit, tried to run away...anything that was a definite rule...home we went.) Essentially, everything fun ENDS when you break big rules in our house. Either for a few minutes, or totally. If we had to leave the park we didn't go back that day (& I'd remind him why), in the house sometimes certain toys (like the one he hit someone with) would go on timeout for the rest of the day...but HE would only be on "timeout" for a few minutes when he was reeeeaallly young, to up to 20 now that he's 6.
By 18 mo I would ask him if he remembered why he went on timeout. These answers are sometime illuminating. More often then not I'd have to respond "No, you're not in here because of X, you're in here because of Y."
By 2, not only did he have to be able to tell me WHY he was on timeout but "help" me come up with solutions and alternatives.
This was when he also started putting HIMSELF on timeout (usually on the stairs or a couch), when he started to feel his temper getting out of control.
This whole process continues to grow and evolve....but yes. You can DEFINATELY put an 18mo old on timeout. They understand far more then you might think.