* Serena and Venus Williams lose to younger opponents
* Champion Serena beaten 6-2 6-2 by Muguruza (Adds quotes)
PARIS, May 28 (Reuters) - It was meant to be a grown-up event for the Williams family at the French Open but now it will be more of a kids' party after two youngsters showed the American sisters to the door on Wednesday.
Slovakian teenager Anna Schmiedlova caused an upset when she beat Venus 2-6 6-3 6-4, but jaws dropped when Spain's Garbine Muguruza knocked out world No.1 and defending champion Serena 6-2 6-2, depriving the sisters of a third-round meeting.
"I think this year there are a lot of young girls who are playing really good. I think (this) is a change now in the top 100, a lot of girls are coming," said Muguruza, 20.
"These things are going to happen some time, a new generation is going to come, and I think now is the moment."
On Tuesday, the 21-year-old Kristina Mladenovic of France confirmed that impression when she beat Australian Open champion and second seed Li Na.
But the youngsters still have a way to go - Maria Sharapova is the last under-20 to win a major tournament, at the 2006 U.S. Open.
Iva Majoli, Martina Hingis and Monica Seles claimed grand slam titles as teenagers in the 1990s, while Serena won her first major (U.S. Open 1999) as a teenager.
"I guess it takes younger people longer to develop. Maybe they don't get to play as much," said Venus.
"Things have changed, but there is still a lot of great talent out there, that's definitely for sure. I think there have been a lot of changes when Serena and I started playing."
It might take some more time before the new generation shuts the door in the face of the 30-somethings, according to Serena.
"When I got close to be 30, I felt like I didn't want to stop. I just wanted to keep going," the 32-year-old said on Sunday.
"Also, technology has changed. Training has changed. People's bodies are better. People are more healthier for a longer amount of time. So maybe that has something to do with it, as well.
"Careers are able to last longer through just modern thinking and modern stuff."
That did not mean much to Schmiedlova, the world number 56.
Under grey skies on court Philippe Chatrier, Venus, the 29th seed, got off to a solid start, winning four games in a row to take the opening set.
Schmiedlova, however, stepped up a gear and, after an early exchange of breaks, stole the nine-times grand slam champion's serve in the seventh game of the second set and levelled the contest when her opponent netted a forehand.
The 19-year-old pulled through for the biggest win of her life thanks to a series of brilliant passing shots.
Since losing in the fourth round of Wimbledon in 2011, Venus Williams has lost in the first or second round of every grand slam she has taken part in, except for the 2013 Australian Open, where she reached the third round.
Williams was diagnosed with an auto-immune disease in September 2011 and she has won only two WTA titles, in Luxembourg in October 2012 and Dubai in February this year.
Schmiedlova, on the other hand, is on the way up, having started to climb the rankings ladder since losing the Roland Garros juniors' final in 2012.
On court Suzanne Lenglen, world No.35 Muguruza showed little deference to Serena, her childhood hero, as she swept her aside in just 64 minutes.
"She said that if I continue playing like this, I can win the tournament. I said, I will try, I will try," said Muguruza, who has a Venezuelan mother and could eventually represent the South American country.
"I will decide in October or November," she said. (Reporting by Julien Pretot; Editing by Robert Woodward/Alan Baldwin)