sutures dont fuse until the 8th decade of life http://www.ncbi.nlm.nih.gov/pubmed/1062937

skeletal maxillary advancement is possible in adults http://www.ncbi.nlm.nih.gov/pubmed/21211241

There are hundred of studies showing it works on children, but only a few for adults. Here are the most significant ones I found:

Facial changes following treatment with a removable orthodontic appliance in adults. http://www.ncbi.nlm.nih.gov/pubmed/15641449

Experimental and postexperimental effects of posteriorly directed extraoral traction in adult Macaca fascicularis http://www.sciencedirect.com/science/article/pii/000294167990277X

Expansion/Facemask Treatment of an Adult Class III Malocclusion http://www.jaypeejournals.com/eJournals/ShowText.aspx?ID=3645&Type=FREE&TYP=TOP&IN=_eJournals/images/JPLOGO.gif&IID=287&isPDF=YES

Continuous forces are more effective than intermittent forces in expanding sutures http://www.researchgate.net/publication/40869669_Continuous_forces_are_more_effective_than_intermittent_forces_in_expanding_sutures

Bone remodeling to correct maxillary deficiency after growth cessation http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3700158/

Age changes in the human frontozygomatic suture from 20 to 95 years. http://www.researchgate.net/publication/22092031_Age_changes_in_the_human_frontozygomatic_suture_from_20_to_95_years

TL;DR - Mewing is a proper oral posture technique that corrects your resting tongue position and betters your appearance