"average ability" is correct and can be used in written English.
You can use it when referring to someone's level of aptitude in a skill. For example, "He has average ability in math, but he excels in science."
How did a person of average ability improve over time?
Children start at the school with average ability but leave with below-average GCSE results.
Hence, an important task of the political system is to select those with above average ability.
"Bright girls are more likely to study physics than those of average ability.
The dotted lines are the high ability kids from each demographic, and the solid lines are the average ability children.
Of the thousands of leaders with whom we've worked, the vast majority possess an above average ability to focus.
Gattis a generation before Arturo would have been familiar with physical and psychological suffering and that above average ability to withstand such traumas seems to have becomes genetic.
"We think we have above-average ability to score runs," Baird said, "but a below-average bullpen.
Year after year, students were entering Tan's class with below-average ability in maths and English, and leaving it with above-average scores.
The Anglo-Saxon countries tend to be more vibrant, less conformist, more open, more meritocratic (although not more equal), than continental European countries - but those things may make them less psychologically comfortable, especially for people of below-average ability.
About 80-90% of drivers believe they have above-average ability, and the more skilled we believe we are at something, the less likely it is to be true.
