I usually wear skates out from the inside. The leather lining is the first thing to go, then perhaps later some structural issues appear like separation of arch support layers on the sole.
I came across a picture of a pair of Riedell Gold Stars from 2006 after I skated in them since 2002. At that time, I was skating at least 12 - 15 hours per week. Note that the leather inside is worn through where it contacts my anklebone. You can see the blue foam padding beneath the leather.
I just took this picture of my replacement skates that I purchased in 2007. Note that the leather inside is shot once again, but this time it's through cracking and old age, not wear.
I skate fewer hours per week now, and this is proof that age is taking a toll on the leather before wear happens.
I hear the phrase "old age" being uttered more and more!