Lindsey, I don't know the answer to that question for sure...but..
X. is just turning 4 and is average height and I just noticed the other day that he looks like he is getting too tall for the toddler bed. If that helps any.
I moved X. to a twin mattress at 15 months because I had one and his crib was broken. He later moved to a toddler bed so the boys could share a room. I moved J. to toddler bed when he was 15 months cause that Teresa offered me her toddler bed. So, currently, I have 2 toddler beds, an unused crib and a dresser in the boys room. (Not to mention all the toys!) We will be moving again soon and when we do the crib is moving into my room (as there will be a baby soon occupying it) and the boys will have a room bigger then their current one. We will get both boys twin sized beds as soon as we can afford it. Our biggest reason is because it's simply more comfortable then a toddler bed!!! Trust me! My kids are not independent when it comes to falling asleep, so I've spent much time laying on their mattresses both pregnant and non-pregnant and it is NO WONDER they wake in the middle of the night and want to sleep in MY BED!!!!!