used growbag compost from 2012

As long as the plants which were grown in it didn't suffer from any obvious disease which may be carried over in the compost it's fine to mulch soil with, although I fork it into my sandy soil because I find that shallow rooted crops like lettuce dry out even quicker if grown through a layer of it.

Don't repeat the growing crop where you have placed the compost.  i.e. if you grew tomatoes in the growbags, don't put the compost where you plan to grow tomatoes or spuds.