My just turned 2 year old is very 'clingy' when amongst others he doesn't know very well. On walls, if people walk towards us be will grab out hand until they walk past. We go to a toddler group and all the other children go up to the teacher and each other to copy what moves she is doing etc (or generally run around 🤣) but he will stay close to me at the back and want me to hold his hand to go closer to the group or to collect a toy etc. It's noticeable as he is the only one that does this (some may be older though but max 3 years old).
He will be starting pre school next September at 3 and I have no idea how I'd be able to leave him there without him getting into a state! We go to various groups/stay and plays etc and he's is ok staying with grandparents without us.