As a parent you watch your kids go through phases, just you aren't always aware that you are entering a phase or leaving one. For example, when is the last time you actually change a nappy? When was the last time you used the buggy to cart them around? You probably don't realise it at the time. One day you do it not realising it, and then the next day you forget to take the buggy out, then you realise that you didn't really need it and then you don't bother... unwittingly that day was the last day.
When was the last moment I wrote a line of Korn Shell, PERL, Java or VB6? Once upon a time those languages dominated my daily life, and then one day, unknowingly, it was the last day. When will it be the last day that I actually turn a line of C#? Will it be because my colleagues can't take it anymore? Or will it be because, by chance, I find myself on an engagement that forces me to use some special functional programming language to solve a massively parallel computing task and then I never end up with the opportunity to use C# again? Who knows. I wonder what that line of C# will look like? Probably just a closing brace and I won't even know it.