Use 'mark you' to emphasize a trait that contrasts with what was stated just before.

Emphasis on Age

'Mark you' introduces a point that may surprise the listener, providing a twist to the sentence.

Unexpected Qualifier

Use it to provide additional information that gives perspective or balance to a statement.

Indicate you're about to reveal something surprising or counterintuitive with 'mark you'.

Prepares the listener for a disagreement without causing offense.

It is used to stress any point that’s not immediately apparent or is contrary to the previous statement.