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When you get older, you become sharp-eyed.
Literal
Year [object-を] take [conditional-と] sharp-eyed [target-に] become.
年を取る ('to age, get older,' literally 'to take years') is the standard idiom for aging. The conditional ~と here marks a general truth ('when X, then Y'). 目ざとい ('sharp-eyed, quick to notice') is an i-adjective for someone who spots small things — the ~くなる form with 目ざとい gives 'become sharp-eyed.' A folk observation about aging.