Baby beds were made for babies and toddler beds were designed for toddlers. That means the weight of the child can make a huge difference in how long the child can sleep on that mattress.
In my honest opinion, yes, I know I'm weird, I have a lot of years in child care and I rely on my education and experience when I share some things. Such as toddler beds and what being a toddler means. Soap box....lol.
The baby's room must be completely baby proof. Shelves attached to the walls with L brackets and anchored into the wood frame behind the wall material. It has to be solid. Then any small toys from you, dad, or older siblings cannot be out within reach in any way. No Barbie shoes or hangers, no Hot Wheels cars, no Tonka Trucks with sharp rusty edges from being outside in the sand box in the rain, simple things most parents do naturally. But the room needs to be looked at through the eyes of a toddler.
A toddler is a child that is learning to walk, toddle around. Once they are past that stage and are walking and running well they are no longer a toddler, they are pre-school age both physically and cognitively.
A toddler is going to get up and wander around, go in the bathroom if that door isn't baby proof, turn on bathtub faucets (Let's face it, bath time is fun time), go through the garage door to go play outside, etc....IF they don't have a closed door or heavy duty baby gate across the opening. A baby monitor in the hallway directly beside their door where the slightest noise in that room will wake you up.
When the room is ready the toddler is ready to move to the toddler bed. We start infants around age 10 months to sleep on cots. So that when they move to the toddler room on their first birthday they are already used to sleeping on a cot and the move transitions better.
With my grand kids I put them in the toddler bed not too long after they turned 1. The latest was 16-18 months. All the rest were will within 33-14 months old range when they moved.
I followed what we'd done in child care even though I wasn't working in that field anymore. I figured that they need to be able to move and grow and have that independence before they figured out what that independence could mean. Our kiddos never wandered. They got in bed to go to bed. They laid down, we read and did bedtime, they were often sleeping before we left the room. Put the gate in place, turn on the monitor if it's off, go off and do our thing.
We didn't have any problems with them wandering but I still took precautions on bathroom doors and kitchen cabinets and cleaner stuff. Just in case the kids got out and into the house area.
It's pretty safe for them if you have it set up to protect them and to alarm/wake you up if they're up.