The issue with Sharapova is that no one is surprised when she loses early in slams. She lost in the 4th round of 3 slams this year, and so her case is different from Serena's. This year, Bouchard, for example, has been more consistent, reaching at least the semis of all 3 majors and with a good chance to make another one. Bouchard, of course, is yet to win a major.
As for age: well, she is 27. At this age, Na Li didn't win yet any slam, and now she has 2 on 2 different surfaces and 2 more finals. Plus, the problems of Sharapova didn't start after, say, the age of 25, but way before: she didn't do much in 2010 or 2011 even though she was 23 and 24 respectively. Her 2010 GS results: 1R, 3R, 4R & 4R. Her 2011 GS results: 4R, SF, F, 3R. The 2011 results are not bad for some girls, but if you are a player like Sharapova who is always in the news, these results are not good at all.
I also think that since 2007 she started to lose at lot and early: in 2007 lost at the 3R an 4R of W and USO, respectively. In 2008, she lost in the 4R of FO and the 2R of W, before getting that shoulder injury. That year, at least, she won the AO.
BTW, during 2010-14, she met Serena only twice in GS matches (W 2010 & FO 2013) and so the problem is not her H2H with Serena.