As a volunteer (or WWOOFer as we call them) you will live alongside your host helping with daily tasks and experiencing life as a farmer.
So you really need to think about which countries might specialise in your preferred activity...
You might want to take child friendly food with you too!
The checks made on the hosts vary depending on where you WWOOF.
Most WWOOF groups use the lower age limit of 18 years old as in most countries people below 18 are considered by law to be children.
The good news is that different rules apply in different countries and so some WWOOF groups do take younger people.