Would anyone have some tips on how to make an old man in his 50's+ be an assassin? An entire life of successful killing, lvl 11, extremely prepared Alfred the butler-like killer. Also acts as a good steward/castelan when needed. Something to give a sorcerer, alchemist and melee ranger a good challenge as a decent boss fight. He can supplement his failings with extreme amounts of consumables so as to leave the least amount of treasure behind if he should ever die.
It's hard to make a martial combatant who is older given the detrimental effect to their physical stats. What about a Dread Pirate Roberts approach where the actual assassin switches out every few years but the name and infamy continue? Just a thought, but probably not what you had in mind.
I have trouble with rogues/assassins as primary melee combatants unless they have a good deal of support. As solo villains I find them challenging to build. The penultimate assassin from Forgotten Realms sprang to mind at your description, though. Have you looked into Artemis Entreri's background and build before? A quick Google search should help. He was an effective melee combatant, but I believe he had a healthy dose of fighter levels to get him there.
Of course, Pathfinder has Red Mantis Assassins, but I have never built one so don't have any insight there, sorry. I hear they are pretty cool, though. Good luck!
Rogue/Fighter/Assassin can also work well. The main thing is he is currently employed by the one of the players as a kindly jeeves/alfred like helper and do-all in his castle, and has been for 6 years. Note that the player never asked how such a refined and highly qualified man got to work for a newbie baron like him, things don't work out and he never truly checked them.
Assassin as been honestly serving the player, doing his best to be a good castelan/steward (or whatever the term is, the guy who handles everything in the castle and then some), but when he receives the go from the one who hired him, he's too to his best to disturb the kingdom, kill council members, etc.
He will get the go at the worst time, when a dragon attacks the city while he is busy outside the castle. Still wishing to do his job and with the players nowhere in sight he will off a couple of members, f+*@ a few things up and run off, only to be met later as a boss fight, perhaps with a few weaker guys backing him up.
He will casually taunt them in fights, attack weaknesses and all around be oddly kind despite trying to kill them. This was supposed to be his last job before he retires in the shadows, so if he dies, he will be a good sport about it and give the players a clue on where he is from.
Since you've mentioned rogue and assassin (perhaps you mean the PrC, perhaps not), I'm assuming you want to rely on sneak attack as a damage source, rather than high strength. Since you want your character to be old--well, middle-aged, rather than really senescent--transferring your to hit to a mental stat is an excellent idea.
I would say that the easiest route is wisdom. You can either dip into monk (sensei) for two levels to allow wisdom to supplant strength with monk weapons or cleric (or life oracle) for one level to get guided hand. The monk route might be nice, since you'll also be able to add wisdom to AC when unarmed, which might benefit one dissimulating under the guise of a butler. Then again, levels of cleric AND monk (with an archetype that does not abnegate flurry of blows) would allow you to make extra attacks via crusader's flurry.
If you decide to employ unarmed strikes (again, being unarmed and unarmored can be beneficial for one disguised as a servant), there's always the Bludgeoner/ Sap Adept/ Sap Master route for more sneak attack (as a monk, you may not even need Bludgeoner).
Sample build outlines (I assume old human, so -3 to physical scores, +2 to mental):
Sensei Route:
Monk (sensei) 2/ Rogue (I don't know the archetypes well) 3/ Assassin 6
Initial Stats:
Str: 13; Dex: 13; Con: 13; Int: 10; Wis: 18 (+2 human); Cha: 11
Eleventh-Level Stats:
Str: 10; Dex: 10; Con: 10; Int: 12; Wis: 22; Cha: 13
If you notice, I'm not a fan of dump stats, but there would be nothing wrong with decreasing charisma to have some constitution or something.
With this build, you want to maximize sneak attack, so you'll take Bludgeoner (if needed; I'm not sure how the monk's ability to do non-lethal damage at will interacts with sneak attack), Sap Adept (probably at level three), and Sap Master (at level seven) as the core of your damage. Death attack and the like will be nice to have.
Master Spy might also be to your liking for this build.
Or, since Red Mantis Assassin was mentioned above, there's this odd thing:
Cleric (Achaekek) (Crusader) 1/ Ranger 6/ Red Mantis Assassin 4
The stats would go much as above, but I'd be inclined to dump intelligence, as in 7 int to start. This allows you to start with 14 charisma, which will increase to 16. You could do Eldritch Heritage with that, if you were disposed. I do hate dumping, but it might work for this character: in his/ her youth, a callow, unsophisticated brute, but he/ she has grown into the urbane infiltrator he/ she is today. That's how I'd justify the low intelligence, at least.
At first level, you take Weapon Focus as the Crusader bonus feat, then Channel Smite and Guided Hand as the human and first level feats. You use ranger levels to qualify for (Improved) Two-Weapon Fighting with low dexterity.
In contrast to the above, you'll have more BAB and some spells from Ranger and RMA, but far less sneak attack. Favored Enemy, Weapon Specialization, and, if you can get it, Power Attack will have to augment your damage. You'll also have to lug sawtooth sabers around, so ... giant cane? In retrospect, this one may not fit your concept at all. Sorry.
Firearms.
Think Boondock Saints.
With their advanced age, he/she has surely acquired some by now, despite their rarity on Golarion.