Most dogs do it, young or old and it often makes them vomit. It is said they eat grass because they feel unwell and supposedly being sick makes them feel better.
I think it is a confirmed "old wives tale" that dogs eat grass to self medicate.
Ive read up on this many times over the years and as heathfield said, "no-one knows for sure" Personally I think they just enjoy the taste or/and the texture.
one of our wolfhounds is 11 months and she loves to eat it.