My 3 older daughters all went into a toddler bed just after their first birthday. We put it up in the room next to the cot so they all had an option. For us, that worked really well and we never had any unsettled nights nor did they want back in the cot. We decided to do it earlier as I was always told the earlier you do it, the easier the transition. Around the age of 2 they went into a normal bed.
DD4 had a few medical issues so we decided to leave her in her cot longer but around 14 months (we were in temp accommodation waiting on our house) she refused to sleep in the portacot so went straight into a single bed without a problem.