Oh I don't know, many people have commented Indy is a different character in all three films, and in reality, he is.
He's a much darker and grittier character in Doom than he was in Raiders(some have said he more resembles Han Solo in Doom) while in Last Crusade, he is a clear good guy who moralizes his actions(i.e. "It belongs in the museum!") He even starts to become more superhero like in LC(complete with a backstory). It's a little hard to believe the lead in either Raiders or Temple would go on an adventure to see his Dad.
I'd like to think if there was one thing everyone could agree with in regards to Kingdom of the Crystal Skull, it was Harrison Ford's strong performance. The #1 concern many people had before Kingdom was released was Harrison's age. He got a lot of flak for it, but he pulled it off. He was every bit as good as he was in the original three. Despite being 65 when he made it, he looked 55 if not younger. The scene where Indy is in Peru in particular, he could honestly pass off as being his 40s. Ford did many of his stunts(and did them well) and had far more enthusiasm for this role than any of his other roles in a while.
I also liked that we saw an older Indy as it brought a great fish-out-of-water theme to the story, and there were some fun jabs/subtle story moments addressing his age but didn't address his age into the plot full-on like say in Rocky Balboa. To say it was just "Harrison Ford pretending to be Indy" and that in the original trilogy he "was Indiana Jones" I think is being unfair(with nostalgia influencing that opinion).
Though having seen Cowboys and Aliens, I do agree that in the 3 years since Kingdom has been released, Ford looks noticeably older. And considering a script still hasn't been written(or even an idea that's been finalized), there's no escaping that his age will be more of an issue in Indy 5 than it was in 4. But, if done right, it can still work well or at least be fun. I don't believe his aging has destroyed all/any potential in this film.
