How old are they when they start to learn Chinese? Do their parents speak Chinese?

我问过的美国孩子，他们都说汉字难学！Maybe their mother language is English and it make it harder for them to learn Chinese.

If you wants to compare the difficulty of language, you need to find two groups of people.
Group 1: mother language is Chinese
group 2: ..................English

Then start to learn a new language in the same age and spend similar time and see how do they feel. I doubt this experiment can be done properly. Maybe it can be a project for students or researchers in social science.