In Australia e.g. it is allowed to volunteer, but you are not allowed to volunteer in e.g. hostels and other accommodation business if you are older than 35 years. If you are 35+ you have to choose work an Australian citizen would not take, like helping in the household. Note that hosts, who wish to have volunteers are looking for help for almost a week, often for much longer time. I decided to ask for home-stays only and I have got lots of hosts – more than I need. I did not know, when I started requesting places, that so many people will be keen to welcome me.
What means volunteering and home-stay? For the first: There are several sites for volunteering all over the world. By most of them, you volunteer for free accommodation and free food. If you are in organic farming, you can become a “wwoofer”: http://www.wwoof.net/. If you like other kinds of volunteering you can sign up on https://www.helpx.net/. For both and other sites you have to pay a small amount for a year or two, because there are expenses to run such a site, but it is worth it! There are also sites where you have to pay for to be a volunteer. There are most about protecting some species, like elephants etc. My site is for budget travels and therefore I do not name the last ones. If you want to help such projects, just google on the Internet.