I wouldn't aim for "exact" per se, but it comes down to having a solid brand positioning first and foremost, aimed at those you WANT to reach, that you feel are the best product/service-to-market fit for what you built your business for in the first place. This won't always be "clean" demographic categories, but rather they may very well overlap into what we focus on here: unique buying tribes (especially if you are trying to reach a large segment of Millennials, who are highly tribal in their nature, digital and offline).
So it may be "oldest half of Millennials who are pet owners in the midwest", or "Retired Baby Boomers who are also homeowners in Southern California", and so on.
Having this set out at the beginning of launching a new startup, or relaunching an existing business into the market with a heavy marketing push, will you help fast-track your marketing and advertising efforts immensely as you'll know more "exactly" who you're aiming at, and it'll be very helpful when picking targeting criteria on whatever marketing/advertising channels you're utilizing (Twitter, OTT, programmatic, YouTube, Google AdWords, and so on).
Of course, throughout all of this will be lots of research...you can try your hand at that on your own using solid resources like Pew Research (https://www.pewresearch.org) and/or starting with Google, which will help guide you to many other solid resources (dependent on the quality of your searches of course).
More than likely, with well (but again...they won't be "exact") defined personas and sticking to 2-3 people groups (not 10), you'll have plenty of folks you can market to, probably more than your marketing & advertising budget can stomach :-)
Happy to discuss this further with you, so feel free to reach out any time!