Define serious. No, don't, but that's why I don't know how to answer this. I was writing stuff as a teenager, as were we all. Those I would describe as earnest attempts more than serious attempts. Does "serious" mean "I will try to get this published?" Or is it, I will finish this damn thing?
My first serious writing was academic writing, but the OP specified creative. I wrote a short story when I was in my 20s and submitted it to a print magazine (rejected, naturally), but I only did that one thing and did nothing more for over thirty years. So was I really serious? I'm not sure. So, even though I have written, and even published (non-fiction) material for almost fifty years, I'd say I only got serious back around 2005, when I decided I was going to write a novel and see it published even if that was self-published. It took over ten years. The fact that it took so long and I stayed with it all the way to finished, that's what I call serious. Why so.
My first serious writing was academic writing, but the OP specified creative. I wrote a short story when I was in my 20s and submitted it to a print magazine (rejected, naturally), but I only did that one thing and did nothing more for over thirty years. So was I really serious? I'm not sure. So, even though I have written, and even published (non-fiction) material for almost fifty years, I'd say I only got serious back around 2005, when I decided I was going to write a novel and see it published even if that was self-published. It took over ten years. The fact that it took so long and I stayed with it all the way to finished, that's what I call serious. Why so.