Philipp Laude and his neighbour Matthias Roll were 15 years old when they made their first YouTube clip: a shaky parody of the scene in Cast Away where Tom Hanks starts talking to a volleyball, which descends into an absurdly drawn-out water pistol fight.

Something clicked, and so did more than 6,000 users: "We suddenly realised: that's half of the population of our hometown," said Laude.

Eight years later, Laude, Roll and schoolfriend Oğuz Yılmaz are the most successful act on Germany's fast-expanding YouTube comedy scene. Via their channel Y-Titty (a play on YTD, short for YouTube Dummies), they churn out at least one new clip every Friday afternoon to almost 3 million subscribers.

These clips include film parodies, computer game voiceovers and comedy songs. One of them, Halt Dein Maul, entered the top 10 of the German charts, and at the end of last year they went on their first national stadium tour.

Understandably, the big TV networks have been knocking on Y-Titty's doors. But if the three twentysomethings were at first slightly embarrassed to point out that they could earn more via YouTube ads (1,000 clicks are roughly worth €1.50 (£1.24) and some of the site's biggest stars are rumoured to earn six-figure sums), they now turn down offers on principle.

"'This is TV, you'll get famous', they told us," said Laude. "But on YouTube we can do what we want."

Musicians and comedians may be using YouTube to kickstart their careers on YouTube all over the world, but there is a uniquely German dimension to the rise of Y-Titty, iBlali, LeFloid, ApeCrime, Alberto or Gronkh, most of whom are based in Cologne.

"Of course the success of Germany's YouTubers has to do with the poor offerings on terrestrial TV here," says Y-Titty's manager Christoph Krachten, a former TV producer who now runs his own science show on the network.

"I don't watch German TV any more," Laude adds. "I keep on trying, but I can't."

With an annual budget of €8bn (£6.6bn), its two main channels may be the richest public service broadcasters in Europe, yet German television has for years been struggling to shake off the impression of an industry in deep crisis.

Middle-aged viewers are deserting national TV in favour of American, British or Danish dramas. Among the educated bourgeoisie in the big cities, not owning a TV set has become a matter of pride.

"German TV is caught in an age trap," said Hans-Peter Siebenhaar, media correspondent for Handelsblatt newspaper. "The average age of German TV viewers is 60. Producers are trying to rejuvenate their output, but we lack the innovation culture here that you find in Britain, Scandinavia or the Netherlands."

One of the few remaining success stories came to an end last Saturday, when it was announced that the entertainment show Wetten, Dass ... ? would be shelved at the end of the year, after 33 years on German screens.

Even the public broadcasters' private competitors are facing declining audiences: only 1.66 million 14- to-49-year-olds watched the most recent episode of the German equivalent of Pop Idol, Deutschland sucht den Superstar. Young viewers in particular are deserting Lagerfeuerfernsehen or "bonfire TV": entertainment shows that could bring together the entire family. According to research carried out by the networks, 14- to 19-year-olds spend two hours less in front of the TV than their parents, on average.

Instead, they turn to YouTube, which is used by 86% of 14-year-old Germans, and where the humour is more in keeping with teenagers' tastes.

It is "a bit bawdier and a bit less polished" than what you see on TV, said Laude. There are few genuine punchlines, lots of toilet humour and plenty of silly voices.

Women are rare on the most popular channels, and if they do appear, they often wear bikinis. And product placement is rife. Acts like Y-Titty are open about making money this way, with certain brands of soft drink featuring with noticeable frequency. Unlike most TV channels, YouTube has no rules curbing surreptitious advertising.

"Our advantage is that we don't have to focus group our target audiences – our target audiences make the programmes", said Krachten, whose multi-channel network Mediakraft manages and cross-promotes most of the key YouTubers in Germany.

"I don't think that in the entire history of television there has ever been a programme like DieLochis, which doesn't just star 14-year-olds, but is also produced by them."