A best practise on this forum is to upload a sanitized sample file. That way the given advice is more straight to the point.
Share formulas using code tags, it's under the insert button.
Couple of things I notice:
- "&&" is a shorthand for "AND" statement. "<18" would be enough.
- Also the "=" before "<" is strange.
The thread is slightly confusing to me. Title says count the age, in your text you want to calculate age. That could be the average age is 14,5 years for players under 18. Looking at the formula you try to count rows.