Semantics, turns of phrase or outright confusion?

I was having a chat with someone recently and I said something that alluded to me being in my fifth decade. My friend commented how I'm in my fourth decade, being that I'm in my forties and not my fifties. I countered that I was, indeed, in my fifth since birth to age ten was my first decade. My friend went on to say that birth to age ten didn't count because a person is too young to at that age to know about things (relationship of self to the world, etc) and, essentially, because there are no digits in the tens column until a person reaches age ten.

This got me to thinking about other terms and phrases that cause confusion and the word 'birthday' came to mind; particularity recalling my son's first birthday party, at age one. I remember someone saying "Happy first birthday!" and I knew what she meant. My son had just turned one but, really, it wasn't his first birthday. His first birthday was when he was born, after all. 

Another one that I often have debate over, and which has caused missteps in planning get togethers, are phrases like 'this weekend' or 'next Thursday.' As many of us do, when we get together with work friends on a Monday, we share our weekend exploits with one another. Some of us use the term 'last weekend' and others use 'this weekend.' Well, which one should it be? The weekend did just end so it was the last one we all enjoyed. Or, perhaps it should be thought of as 'this weekend (that just ended).' The confusion continues when, for example, we're talking about what's going to happen in the next week or so. If, on the same Monday for example, discussion of Thursday comes, someone may refer to the upcoming Thursday as 'next Thursday' because it's the next Thursday coming up while others may say 'this Thursday' because it's the Thursday of this (the current) week.

I suppose a lot of how we think of things and refer to those things comes from our own personal preferences and the lingua franca of where you are. But, is there a 'proper' way that the above examples should be regarded? I've often wondered that. Maybe you have, too. If you have a definitve answer or know where I can find one, please let me know. If you have your own vocab conundrums, share those in the comment section.

Thanks for stopping by and have a great, today, this Tuesday and enjoy your time off this coming, next, weekend, whatever decade you're in and have a great birthday whenever it may be.