It does not sound natural to my (American) ear. I would prefer one of these options:
- As children get older, the percentage who own mobile phones rises.
- As children get older, the percentage of them who own mobile phones rises.
As an American, "cell phones" seems more natural than "mobile phones". The term "mobile phones" is more common in other countries. The term is becoming more popular in the United States, partly because many websites have "mobile" options.
I would expect the phrase "of ones who own mobile phones" to be used to restrict a dataset. For example:
American children can be categorized by whether they own mobile phones. Of ones who own mobile phones, 98% are more than two years old.
(I just made up this statistic.) In my example, "ones" is grammatically correct, but "those" is more natural.