We put my oldest son in his toddler sized car bed at 18 months old, and he moved out of it at 4 1/2 years old, so it was worth the $75 for those 3 years, the extra metal grated support, and the same crib mattress and crib sheets (toddler bed is crib size mattress/sheets) that he already had, lol. It was that cute Little Tykes blue race car, in GREAT condition, bought off craigslist. (Plus it was handed down to the next son).
Now at 4 1/2 years old, he's got the twin bunk beds. First, boys don't sleep together in the same bed. I don't know why, we girls did all the time. But my brother, my friends' sons, etc....they just don't sleep together. So maybe as little children fine they can share a bed, but at some point in elementary school that just stops and someone feels uncomfortable about it. Not saying they should, but it is what it is. If he didn't care about bunk beds, we would have gone full size and had room for a dresser, but he has the T shaped bunks with book shelves, a headboard with 2 shelves, a dresser, 3 drawers on the bottom bed, a desk, and a cork bulletin board all built in together. We couldn't pass that up! It works beautifully in his room, and he has a display shelf for trophies and the collectables, and a toybox in his room with plenty of room to play in. That wouldn't have happened had we had to buy a full size bed and separate dresser. Besides, all that rustic wood goes beautifully with his cowboy themed room but could change to a "cabin" or "pirate" room easy enough in a few years if he wants to change his theme. He's still small enough that I can lay on my side in his bed and cuddle him (and little brother somehow squeezes in too!) But the chair for his desk is what I use if I need to sit down and talk to him. Incidentally, I got the bunk bed on craigslist from someone who's room was too small for it, and paid $130 for it. Just bought 2 new mattresses during a memorial day sale and that was that. Someone else made a good point: twin beds are what you get if you stay in a dorm. Or the army. So no, a quality twin bed is fine as far as "size" goes.
Another mom posted about being a better sleepmate later if you sleep in a twin. I laughed at that! Reason: I had a king sized water bed starting at age 8, and I AM the worst sleepmate in the world. I roll, flip, flop, sleep diagonally if possible, etc. My husband grew up in a twin bed, and he doesn't even wrinkle the sheets! He just lays there, and sleeps. There might be something to that theory, Jae.
Now my youngest is 19 months old and LOVES his toddler sized car bed (handed down from big brother) and his transportation themed room which is so cute it's ridiculous. We keep a beanbag chair that matches one of the colors in his quilt by his bed so I can flop down and read, pray, take temperature, and all that mom stuff.