I'm guessing here that Phoebe married George Cooper in Worcs 1825?
In the 1841 census ages were rounded off, the son may have not been 15.
Have you found a birth for Joseph?
There is a bapt in IGI for Joseph Thompson Cooper 1827 mother Phebe
St Thos Dudley Worc.
I'm not sure if this is of help.
oznannie