They need the most energetic, charismatic, vigorously dynamic, larger-than-life speakers and leaders that can possibly be found!
Not every single person involved with speaking to young adults or leading young adult groups has to be a human dynamo of "youthful" pizazz and zippiness. To try to acheive this is a mistake.
Now, there is nothing wrong with having such people involved. Their energy and enthusiasm can be a great help. But not everyone involved needs to be like this.
I speak here from direct experience. Young adults will indeed listen to and respond to speakers who are not human Energizer bunnies. What matters more than sheer energy is that speakers and leaders
1) deeply care about young adults (whether they still be one or not) and like being around them,
2) can identify with and empathize with young adults and their needs,
3) have basic public speaking skills,
4) know and love their Catholic faith and can communicate their faith with hope and joy,
5) are authentic,
6) for those in leadership, that they have basic leadership skills, and
7) pray for and with those whom they serve.
To expand on no. 3, it is fine if a person is not necessarily the greatest speaker in the world. But they do have to be at least decent--capable of being understood and of communicating ideas with clarity and conviction. As important as they are, style and delivery are not everything. A mediocre public speaker who is extremely sincere and honest and has a great message can still be very effective. Also, naturally quiet, introverted people can be excellent public speakers (in fact, many Dominicans are actually introverts by nature). One does not have to be a "force of nature" with a great deal of charm and gregariousness to be able to hold the attention of an audience (the joy of no. 4 can be quiet joy, which can have its own special power). I have seen the reality of this demonstrated many times. And besides, no matter how great a speaker is, it is not likely that he (or she) will be able to pluck the heartstrings of every audience member. Different speaker personalities will effect people in different ways. Considering the audience, quiet, introspective types can even be put-off by highly exuberant personalities. So it is a good thing not to always seek the dynamos as speakers.
A good variety of speaker personalities with consistently high quality, meaningful, relevant content, coming from earnest and loving hearts--this is what will work over the long term. Maintaining a standard of good public speaking need not translate into presenters that all have the same style.