LinkedIn announced its third-quarter earnings yesterday and boasted a 38% increase in membership on the year. The professional network is now 259 million users strong and growing. While LinkedIn’s growth is what developers and investors are looking for, the new user-base may not be coming from lucrative sources.

CNET pointed out that LinkedIn’s increase in users could come from an unexpected source: students. The professional network has been ramping up its university pages to connect alumni with current students at their Alma mater. LinkedIn has also been promoting its university pages to high school students and branding them as a place for aspiring attendees to talk to advisers, connect with professors, and learn more about the school.

LinkedIn is catering to both starving college students and high schoolers, but that’s not enough for the network. Last month it lowered the age limit to join to 14 in the US and 13 in the UK. This means that as soon as kids hit the legal working age (14) they can start beefing up their resumes.

From 14-18 they’ll highlight whatever experience they can get their hands on (President of the debate team, bag boy at Wal-Mart, dog sitter) and then transition their profiles to match their fields of study once they reach college (President of the college AMA chapter, intern, sorority event planner).

What’s the problem with having a bunch of kids on your site? Is there a problem? Facebook has kids and it’s doing just fine. Heck, Tumblr is thriving and 32% of its audience is younger than 25.

The problem lies in LinkedIn’s plan to monetize via ads. Business Insider reported that some of the most successful ad campaigns on LinkedIn were by Citigroup, Cathay Pacific Airways, and Prudential Retirement. Companies that advertise on LinkedIn do so because they’re targeting hyper niche demographics. A social analytics software program can target marketers who are already in a professional networking frame of mind when they’re on the site.

Teenagers and college students pose a problem. It’s hard to serve them ads when their only skill is ‘MLA format’ and only their only management experience comes from German Club. More importantly, this demographic isn’t thinking about retirement, its main concerns are what to wear to prom and how to pay their fraternity’s dues. Serving them ads for credit cards and flights to India is a waste of money for advertisers.

While opening the doors to minors may have helped LinkedIn’s numbers, it may have hurt its popularity and branding. The IB Times reported that LinkedIn’s age restriction was one of the main factors that differentiated it from social networks. It’s a professional network to connect with people who are already established in their careers, not just teens who are testing the waters to see what interests them.

Mostly users fear that this is a downward spiral. Attracting the teenage demographic now might lead to further design elements by LinkedIn developers to appeal to teens – elements that could make LinkedIn look more like Facebook.

Where do you stand on the issue? Is targeting minors a way for LinkedIn to gain new users while helping a professionally-challenged demographic? Or is it an attempt to monetize at the expense of current users?