I've never understood what that line by Sollozzo meant precisely. For me, it means either one of three things, or perhaps a combination:

1) That ten years ago, he could've "gotten to" to Don Corleone--As in, made him "see the way", convinced him to go along with the drug trade. That the Don was too set in his ways now, too old fashioned, but ten years ago he might've gotten through to him and made a deal.

2) That he couldn't have killed him if it had been ten years earlier; At the time, the Turk thought Vito was dead. So he's asking rhetorically, could he have gotten to him ten years ago? Meaning at one time even he recognizes Vito was a force not to be messed with by anyone, but those days were passed and he killed him.

3) That the Don's instincts weren't as sharp anymore--That ten years ago, Vito might've been more keen and smelled a rat with Sollozo's plan and whatnot. That he would've never been able to get close enough to kill him if Vito had been ten years younger.