Realistically, how does the GOP or Dem Party hope to secure young voters?

[pbrower2a]

Young voters will always lean Dem.  They certainly won't be as enthused or active for Hillary as they were for Obama but they will be more enthused for Hillary than say, Kerry or Gore. Young women will probably find the narrative of the first female president to be very enticing.

The youngest voters DID NOT lean Democratic in 1980, 1984, or 1988. Accepting the Howe-Strauss division between Boomers (last born in 1960) and Generation X (first born in 1961), X voters have been more conservative than any other generation of American voters since the Lost born in the latter part of the 19th century (1883-1900).  Except for personal sexuality, they were much more conservative than America at large on economics, the environment, and labor-management relations. Few people saw the extent of the Reagan landslides of 1980 and 1984 as they did and that Dukakis could collapse as he did. Not until the Republican Party started talking about abortion bans did Generation X start looking at Democrats as a viable option.

[pbrower2a]

It seems that Democrats have already secured the 25-29 year old subgroup of the youth vote. If you look at many of the exit polls even from the disastrous 2014 elections, the Democratic candidate almost always won the 25-29 year olds while losing the 18-24 year olds. As someone in the 25-29 year old range (speaking from personal experience), it'll be very hard for this age to go Republican. We grew up during the Clinton years when the economy was strong and then came of age during the Bush era, which made many of us anti-Republican. Obama's [alleged] "coolness" appeal in 2008 helped solidify the current 25-29 year olds in the Democratic camp. 2012 certainly didn't help with Romney going from a Massachusetts moderate to a "severely conservative" candidate on every issue. I think my age group will support Hillary in 2016 by strong margins, inspired by the historical aspect of her campaign but also out of nostalgia for the good ole days. True, social issues hurt the GOP with younger voters, but there's also a sentiment that the current Republican Party is anti-science and the climate change denial doesn't do them any favors when many younger voters do believe that climate change is real. Abortion is always going to be a 50-50 issue. They should fold on marriage equality and stop all the draconian "religious freedom" bills that are passing the GOP-controlled state legislatures, because that's only going to make them come across as more anti-gay and homophobic than they already are with young voters, who overwhelmingly support LGBT rights.

Here are some of the 2014 exit polls that I could find for the youth vote (for the U.S. Senate):
ALASKA
18-24: Sullivan 51, Begich 43
25-29: Begich 50, Sullivan 39 (Begich's best age group)

ARKANSAS
18-24: Cotton 48, Pryor 47 (Cotton's worst/Pryor's best age group)
25-29: Cotton 53, Pryor 44

GEORGIA
18-24: Nunn 61, Perdue 37 (Nunn's best age group)
25-29: Nunn 55, Perdue 43

IOWA
18-24: Braley 51, Ernst 46
25-29: Braley 52, Ernst 45 (Braley's best age group)

KANSAS
18-24: Orman 54, Roberts 43
25-29: Orman 60, Roberts 36 (Orman's best age group)

KENTUCKY
18-24: McConnell 53, Grimes 42
25-29: Grimes 52, McConnell 43 (Grimes's best age group)

LOUISIANA (Pre-Runoff)
18-24: Landrieu 47, Cassidy 38, Maness 12
25-29: Landrieu 53, Cassidy 35, Maness 7 (Landrieu's best age group)

MICHIGAN
18-24: Peters 54, Land 40
25-29: Peters 59, Land 34 (Peters's best age group)

MINNESOTA
18-24: Not Polled
25-29: Franken 61, McFadden 36 (Franken's best age group)

NORTH CAROLINA
18-24: Hagan 47, Tillis 44
25-29: Hagan 59, Tillis 34 (Hagan's best age group)

NEW HAMPSHIRE
18-24: Shaheen 54, Brown 46
25-29: Shaheen 62, Brown 36 (Shaheen's best age group)

TEXAS
18-24: Not Polled
25-29: Cornyn 52, Alameel 35

VIRGINIA
18-24: Warner 54, Gillespie 41
25-29: Not Polled

This bodes ill for Republicans in the 2020s and 2030s, when Republicans will have shown how extreme they are -- and when people now in their late 20s will then start running for high public office. Millennial adults will find their own generation more likely to address their economic and cultural concerns than will old X and Boom pols (unless the X and Boom pols have kept pace with the times) with enriching the 'right people' and continuing the Culture Wars. Educational quality will matter far more than cozy deals between lenders and over-priced, substandard colleges to get students deeply in debt for questionable education -- or with pushing "Abstinence Only" sex education (which does not work), school prayer (one would expect that to be settled now), young-earth creationism, and denial of climate change.

The Millennial Generation is much more amenable to institutions, whether Big Business, government, or labor unions, so long as they are competent, sensible, equitable, and honest. But if those institutions are incompetent, irrational, unjust, or corrupt one can expect the Millennial Generation to have any stake in the status quo.

18-24? They are still in the honeymoon phase with the economic order. They can outgrow that fast with some serious disappointments.