Is it too much to ask a 23 months old to wait for his turn to play with the swing at the playground? Or they are still too young to understand that ?

no, never too young to know! although most toddlers do not have 'patience' because that's how a toddler will react, they do and will understand when you teach them to take turns. My boy goes to the playground often from 14 months onwards and now at 28 months, he will wait for his turns and invite others to play together too. just keep talking to him like he is an adult and explain why he should wait, and the playground is for everyone. however, when my little one was not vocal, after waiting for 5 mins i will step in and help to ask the other kid if he can let my kid play, and normally the other kids will be glad to share and take turns too.