I've done a lot and been many things in my life, but the decision process for going into any of these things was roughly the same. Before I decided to try something out I thought about the pros and cons of doing it. Part of that is knowing the odds of it being a successful endeavor. For example, at one point around 2013 I was seriously considering opening up my own independent video game store. At that time you could still find tons of deals everywhere and I had built up half the inventory I'd need to start up a decent sized game store. I then looked into other aspects of the business before deciding that it wasn't going to work out, at least to have a physical store. However, I gave it a serious thought and looked into the logistics of actually doing this, and it didn't work out. Likewise, I have gone into stuff and it has worked out; I was a social worker for 5-years making barely anything and jumped into IT by obtaining some tech certs on my own time, and now I make over double what I did back then.
It's really just looking at what you want to do and deciding what your odds of success are, what is needed to do it, and how you'll go about doing it. Wanting to be an actor is fine, but going in expecting to become the next Brad Pitt is probably not best mentality to have. However, if you love acting, getting into theater, looking into acting gigs and roles, however small, is a great start.
As far as when to pursue a dream, there is no better time than the present. If you want something go for it. Just make sure you are smart about it and think out the pros and cons like I said before uprooting your whole life to pursue something.