I don't know if this is responding much much much too late, but I can totally relate to this 'transition'. It's an interesting time for everyone!
I've had three boys go through this, and it's the same every time. Because they now can move out of their bed, they do. It's a perfectly natural response, and really new territory for them. I don't think 18 months is too young, but if you're going to stick to it, this is what I'd suggest:
-lay a mattress or something soft next to their bed in case they fall out during the night (mine ALL did, until they got used to sleeping in a bed, and this sometimes took a few weeks)
-every time they get out of bed, put them back, firmly but gently. It seems that if they know they're getting under your skin, it does become a game with them, but if you're calm, they seem more apt to be calm too...
REMEMBER, this doesn't mean they'll automatically stay in bed. In fact, they'll keep pushing the limits over and over and over until they finally realize you're not budging. My youngest is 21 months and he's been in a bed for a few months. He still gets up and wanders 2-3 times. We just put him back in bed; Maybe it's because he's our third, but it doesn't bother me as much anymore.
-Try to negoiate.... he may take a special toy or book to bed, but only if he stays in bed, and if he gets out, he chooses to give it to you for the night, and he can try again tomorrow morning (or he can try again in a few minutes, whatever you can live with)
Just a few suggestions...if it helps, my older two starting staying in bed by the time they were close to 2. I don't know if it's because they had the few months to get used to it, or if some magical button is hit at that point, but the consistency paid off.....hope that helps