How old is too old for diapers?

[Inspired by Reggio]

IMO there is no magic age anymore ... the readiness to train is dependent on so many things!

I start exposing children to the concept of the potty at 18 months and on average children are fully trained by 3 in my group ~ however I have seen children who are over 4 still not trained cause they just refuse and it is a huge struggle and well not worth the stress and battle so back in diaper they go ... and than one day wham they are without any fight or struggle they just wake up one morning and decide they are ready?

If he is not able to indicate the need to go and is willing to SIT in a soiled underwear than ya likely not ready to push training!