Specificity wins in entrepreneurship not broad generalizations. You have to get real customers, real money, real product and real follow on sales. As you mention, all of this happens with laser like focus. “Millennials” is too general. You need to be more specific. 
This is with regard to your beachhead market. With regard to follow-on markets (Step 14), you do not need to be nearly as specific and don’t waste too much time getting specific because it will distract you from winning the “first game,” and if you don’t win the first game, you are done.
So if they ask “Why only Mellennials?”, ask them if there question really is about “how do I make this a big business?”. That is probably what they are asking you if they are wise investors. In that case, the answer it is for a well defined market that we can win and then we will “land and expand,” just like Jeff Bezos did at Amazon. Make sure you tell them specifically who your target beachhead customer is (Step 9 – first ten homogenous customers will go a long way towards showing you have sufficient market and you have de-risk the market risk) and then what the follow on markets will be (Step 14). You need to explain why you will have gain competitive advantage to be able to dominate those markets after you win the BHM (beachhead market) and how big that market it. But disciplined wins the race. Nail it before you scale it. And to Nail It, you need specificity. Lack of homogeneous customers will drive up all your costs, most of all COCA. Stay focused.

Hi Floriano, talking about specificity in your question regards of part:
“Should this also be evident in our messaging that speaks to the market, including potential investors?”
I recommend the focus on your message is you taking only and specifically to your well defined persona.
even though potential investors are important to convince in this funding they are not your customer, nor the final focus they are there help to (and will also have a benefit) so by them understanding clearly the message they will be bought in as they see the pain point is well addresses. 
Good luck!!  Mateo

Mateo! 
Thanks for jumping in. Really want to make this a community website and a place where P2P (Peer to Peer) help and dialogue can thrive.