How long is a generation?

I always thought a generation was the difference in years between parents and children. And also a group of people being born in the same timeframe. What is a timeframe in this context? Is it the normal age difference between children and parents, or is everyone born in the same decade or what? Am I in the same generation as everybody having children in the same age as I have?

When someone older is refering to "your generation", it is a way of pointing out that I'm younger. On the other hand if someone younger is doing the same, it is a way of pointing out that I'm significantly older.

It makes me feel old. Me and my generation.