I don't think you can say it's harder for kids or it's harder for adults. Suffering isn't measurable firstly and secondly transitioning as a child is so much different then transitioning as an adult.
If you're very young I think it's hard because you have less control over your life, you're dependent on family. Also you're still growing, developing... you might be in puberty which is hard anyway, but will probably be harder being trans. But if you get help you'll spend more of your life being yourself, you can fight puberty, you can spend your later life focussing on other things...
As a young-adult, beacsue I think there's still a difference with adult, you have more control over your life but you're also in the middle of doing big things like finding a job, studying in college... That might make it hard or put your life on hold for a while.
As an adult you're likely to have a relationship, a job, children... those things will probably make your transition complicated. However you might have a steady income, savings, a house... which might help you transition easier.
So I think every period of transitioning will have their own advantages and disadvantages.