What is the efficiency of a query + subquery that finds the minimum parameter of a table in SQL?

I’m currently taking an SQL course and trying to understand efficiency of queries.

Given this query, what’s the efficiency of it:

SELECT *
FROM Customers
WHERE Age = (SELECT MIN(Age)
             FROM Customers)

What i’m trying to understand is if the subquery runs once at the beginning and then the query is O(n+n)? Or does the subquery run everytime you go through a customer’s age which makes it O(n^2)? Thank you!

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