Which one is a real paradox?
The first one is arbitrary. 15 years old is young compared to 25 but old compared to 5. So its not really a paradox.
The second one is also not a paradox. A "stack" is not a unit of measurement so its not a contradiction to say that 10, 20, or 10,000 rocks is a stack.
A real paradox would be saying that the curvature of an infinite circle is a straight line.