Ava has just learned to tell her teacher how many markers are in the set by counting each one and saying the last number as the answer to “how many? ” ava is most likely year(s) old.

Explanation:

Ava is a child who probably got admission into the school and has learned to count.

Her age can be between 3-4 as in this age toddlers get admission or they start to learn to basic counting.

She is just counting the number of markers and as per her the answer of "how many" means the last number she has counted. She is probably too little.