Why this is a surprise to some of you, I don't know. It has been obvious for months that WWE want to build Sheamus as the big replacement for Cena, and it's been known even longer than that that Triple H feels Sheamus is the future of the company. I also don't think 35 is too old. Assuming Sheamus retires in his mid/late 40's, he has a good decade or more left. Austin was 33 when he first became big, and the neck injury is what forced him into retirement even then (and only after he already became the biggest draw of all time.) A lot of wrestlers hit their prime within those years. Most wrestlers, even the biggest draws of all time, also don't maintain the same type of drawing power for that long, so it's not really an issue. Hogan lasted about 7-8 years before he went stale in his WWF days. It's not really an issue at all.
That said, I don't ever see (or hope to see) Sheamus as the face that carries the company. He's a WWE Champion, WHC, KOTR winner and Royal Rumble winner and has been wrestling in world title matches for nearly three years now and he still doesn't seem any closer to being the next big thing than he ever has before. I don't see why WWE doesn't just try to make someone like Ryback their next top star since Vince thinks he could be the #2 babyface of the company (behind Cena) as it is and he obviously has the cosmetic look that WWE loves.