slabu -sau- gismu

x1 is old/familiar/well-known to observer x2 in feature x3 (ka) by standard x4.

This can cover both meanings of old. Old in years, i.e. age, can be conveyed through x2 = the world, life, existence (= loi nu zasti); in usage this has been a common default for ellipsis. However slabu is not the opposite of 'young' (= nalci'o, tolci'o), but the opposite of 'new' (= tolni'o); also ancient (= tcesau), age (= nilsau); x2 is used to x1 (= selsau for reordered places); historic/historical (= cirsau, cirselcedra; also vaipru). (cf. clani, citno, cnino, se djuno; not the opposite of citno, djuno)

In notes:

x1 is young/youthful [relatively short in elapsed duration] by standard x2.
x1 is long in dimension/direction x2 (default longest dimension) by measurement standard x3.
x1 is new/unfamiliar/novel to observer x2 in feature x3 (ka) by standard x4; x1 is a novelty.
x1 knows fact(s) x2 (du'u) about subject x3 by epistemology x4.
attitudinal: familiarity - mystery.
s1=c1 is logged into system/application s2=c2.
x2 is used to x1 (note out-of-order places); x1 has become normal or unremarkable to x2
x1 is old (un-novel) to observer x2 in feature x3 (ka) by standard x4
x1 is familiar to the memories of x2 in feature x3 (ka) and by standard x4; x2 knows/is familiar with/has lived memory of x2.
x1 is old/aged [relatively long in elapsed duration] by standard x2
x1 is remembered by someone/something which remembers ... which remembers someone/something which is remembered by x2 via intermediate nodes/steps x3 (ce'o; ordered list).