L.L.
I put my daughter in a toddler bed at 18 months. Starting at that age, her cue to get up was "If the sun is shining" - her bed was next to her window, she could stand on it and peek out the window. If the house was dark or if the sun wasn't out she wasn't allowed to get out of bed. Also, I think she might have been scared to get out of bed alone too. She ALWAYS would call out and say "mommy? can i get up?" and I would tell her yes or no depending on the time / if the sun was out. It worked for her.