Always kept my hair short, so looking back in photos its tough to tell when it started and even looking in the mirror the last 3 years it never stood out to me. But with some photos I started noticably thinning sometime in 2009. It was ever so faint in Sept 2008, but in June 2009 you can see it more clearly. At my 30th in 2010 you can see its not just short hair. In summer 2011 some photos were taken and in October 2011 when choosing photos for match.com I had the "aw crap" realization. In some ways, I wish to god I'd remained oblivious.
Though I will say that the hair loss has accelerated starting around the same time I discovered it