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.

I'm sure he meant that 10 years ago he wouldn't even have got the face to face meeting with Vito and his inner circle let alone the actual hit... Indeed, Sollozzo thought Vito was dead at the time he made the statement.

Sollozzo asks about "getting to him". I'm pretty sure he's referring to the physical.

I believe in a combination of 2 and 3.

Always took it at face value, meaning the hit itself. "Ten years ago", Solozzo wouldn't have been able to take a shot at the Don of the Corleone Family and gotten away with it, unless it was planned as a suicide mission - which, given Solozzo's ambitions, it would never have been. No point at all, beyond some sort of vendetta.

Had nothing to do with Vito's own instincts or abilities, merely a reflection upon how the situation had changed - ten years earlier, none of the other Families would have dared (or even wanted) to upset the status quo. A hit on Vito would have done too much damage to their own fortunes.

Solozzo's actions, and his ability to get away with them, are meant to be an indication that the peace that the all-powerful Corleone Family lorded over was at an end. The final part of the first film is about exactly who was upsetting this peace, and how the Corleone Family dealt with this upset, which they did.

That said, the circumstances around the hit on Vito are also something of a commentary upon how complacent the Corleones may have become - the Emperor of New York is shot almost dead while buying fruit from a street vendor, with only his weakest son as a bodyguard, one who fumbles his gun in a pathetic and worthless attempt to defend his father.
There's that shot where Vito suddenly becomes aware of the danger, but this is a man who has had men trying to kill him since he was an infant. His offspring, born in the land of opportunity, are all too stupid, weak, straightlaced or hotheaded to deal with a real war.

All but one, of course.

I believe he meant 2 - the hit - but that also encompassed some of 3 - Vito's diminishing instincts which led to the circumstances of the hit (sending Luca on a fool's errand, not thinking to protect himself, etc.).

By "got to him," Sol meant, "killed him." Before he said, "10 years ago..." he said, "Let's face it, Tom, the Don was slippin'. That tells me that 10 years earlier, Vito, having recently fought a life and death gang war (novel) would have been a lot more careful about being bodyguarded, and about being careful of whom he dealt with.

IMO, the real slip Vito made was sending Luca to Tattaglia with that ridiculous story that he was "not too happy" with the Corleone family.

I concur.