“hath outstretch'd his span”
'Span' is the measured length of a human life — from the Old English 'spann,' a hand's breadth used as a unit of measurement. To say Timon has 'outstretch'd his span' means he has lived out his allotted time and is now dead. The soldier is reading the epitaph and relaying its claim.
“the character I'll take with wax”
'Character' here means written letters or an inscription, not a person in a story. The Soldier cannot read the script on Timon's tomb, so he presses wax against the stone to take a physical impression of the letters and carry it to Alcibiades, who can decipher it.
“hath in every figure skill, / An ag'd interpreter, though young in days”
'Figure' means any written character, letter, or numeral — not a person. The Soldier praises Alcibiades as someone skilled in reading every kind of writing, and calls him 'an aged interpreter though young in days': he reads with the wisdom of an experienced scholar despite his youth.
- TLN 2315classical allusion
“Before proud Athens he's set down by this, / Whose fall the mark of his ambition is”
Alcibiades has besieged Athens — 'set down before' a city means to encamp an army outside it for a siege. The Soldier reports that Alcibiades has already taken up position at Athens's walls by the time the Soldier finds Timon's tomb, and that Alcibiades aims to destroy the city that banished him.
classical Alcibiades and ungrateful Athens