Maths Question of the Decade

In about October time, someone twigged on that the decade was coming to an end. All we've heard since is 'decade', 'decade', decade'...

As it's my daughter's birthday, my mind shifted to a possible maths problem... (in a similar fashion to the Birthday Riddle): 

So, I'm 36 and I'm entering my 5th decade. 36 and 50 (ie 5 decades) are a fair distance apart. The children currently in Years Four, Three, Two... are only just entering their second decade.

So, here's the question:

How long would someone have to be alive to enter their second decade at the earliest opportunity?

How long would someone have to be alive to enter their second decade at the latest opportunity?

How long would someone have to be alive to enter their fifth decade at the earliest opportunity?

How long would someone have to be alive to enter their fifth decade at the latest opportunity?

How long would someone have to be alive to enter their tenth decade at the earliest opportunity?

How long would someone have to be alive to enter their tenth decade at the latest opportunity?

Or, any other similar questions that may come to mind...

What's the longest a decade can be? What's the shortest a decade can be? (NB: Leap Years ;-))