Translation:We are in the same city thirty years later.
Could be 'we have been in the same city for thirty years.' I seem to remember that italian uses the present tense to express a period of time running to the present, e.g. 'I am learning a foreign language for five years' instead of using the past tense as we do in english.
Adjectives with other characteristics normally precede a noun. Those characteristics are beauty (bello, brutto), age (nuovo, vecchio, antico), goodness (bravo, buono, cattivo), and size (grande, piccolo). An easy mnemonic to remember is BAGS: Beauty, Age, Goodness, Size.
Examples: una brutta macchina (an ugly car); un vecchio libro (an old book); una brava ragazza (a good girl); and una grande nave (a large ship).
Sometimes the position of an adjective is changed for emphasis.
You are correct about using "lo" before certain words starting with s+consonant, gn, z, pn, x and a few other letters, but this rule is for masculine words and does not apply to words which are considered feminine.
In this case you use nella because the word città is considered feminine (la città). If it were a masculine word such as paese (il paese), you would use nello (nello stesso paese).
Siamo NELLO stesso paese, ma non NELLA stessa città. = We are in the same country, but not in the same city.