Please, explain the use of 'would' in this sentence

Question

Please, explain the use of 'would' in this sentence

I'm only 36, but I feel like I'm too old to understand and explain certain expressions sometimes.. (since this is something younger people would usually say)

Answer 1

That use of "would" appears in this definition:

American Heritage Dictionary "would"
"6. Used to express presumption or expectation"

The speaker believes that some expressions are most likely to be said by younger speakers. (They take that as a reason for not bothering to understand or explain the expressions.)

Let me suggest a corrected version of your sentence:
"I'm only 36, but I feel like I'm too old to understand and explain certain expressions, since they are things younger people would usually say."

Answer 2

In this case, the word "would" suggests a presumption of an event occurring based on common knowledge. In fact, I would go further and say that its more of an assumption.

Here, the speaker is assuming that he is, for the lack of a better phrase, from an older generation, which used different phrases and expressions and as a result, might not be able to understand every colloquial slang used by the people he's referring to; unlike the more common usage "used to".