For most attacking wide players, yeah. But Sanchez is a special player, one who would be the first world-class wide player we've had since selling Ronaldo. For a club who traditionally prided itself on their wide attackers, it's odd that that scenario has been allowed to play out.
Personally, I don't think Sanchez's age is an issue - he's phenomenally fit, hasn't shown any sign of being in physical decline and would improve us immediately. Nedved moved to Juve at the same age and went on to become a club legend for them. I'd look at that transfer in relation to this it if it comes off. He's got at least 3 more seasons playing at his high level, and his joining us would massively weaken a top 4 rival's chances of qualifying for the CL. Say the transfer ends up costing us £30m, and we get those number of seasons out of him, you're looking at £10m per season for Sanchez. That's an insanely good deal, especially in the context of the current transfer market.
To apply the words/logic/philosophical musings of LVG to this case - "WE GOT FOR IT!"