I assume you are referring to private pensions.
If so, yes it is possible (subject to scheme rules) for someone to retire early on ill-health grounds from an Occupational Pension Scheme. This is something that normally has to be agreed with your Employer. If agreed, then you draw down the benefits as if retiring normally- though they will obviously be lower because of early retirement.
If on the other hand you have a Personal Pension (typically self-employed) then you cannot draw down the benefits until age 60.
Hope this helps.