Shelley Mullane's reply:
The good news is that you have already started! By volunteering in several museums you are gaining valuable skills as well as relevant experience to put on your CV. However, the heritage sector is quite broad. You should think about which area of the sector you would like to progress in and make sure your experience, voluntary or otherwise, fits in this area.
I didn’t start working in the heritage sector until my late twenties. I focussed on visitor services and retail as I had skills in these areas from previous positions in high street chains. You’d be surprised at how many transferable skills you have.
In terms of placements I would, again, ensure that it fits within your chosen area. There are placements and internships out there for all ages. Demand is high and competition is stiff. Ask in the museums you are volunteering in, sign up to listings or check listings online. Here are some good sites to try, that include both paid and voluntary positions: University of Leicester Jobs Desk: http://www2.le.ac.uk/departments/museumstudies/JobsDesk , Museums Jobs: http://www.museumjobs.com/ , Arts Council, Arts Jobs: http://www.artsjobs.org.uk/ , Guardian, Arts & Heritage Jobs: http://jobs.theguardian.com/jobs/arts-and-heritage/ With all of the volunteering you are doing you should be able to get a good set of references and letters of recommendation too.
I wish you luck in your chosen career path. Working in the heritage sector can be very rewarding and an endless source of inspiration.