。
At this rate, it looks like you'll live to 120.
Literal
This-as-[if-と] 120 years-old [until-まで] live-[looks like-そうだ][emphatic-よ].
このままいくと 'if this continues / at this rate' is a set phrase built from このまま 'as is' + いく 'go' + と conditional. ~そうだ after a verb stem (here 生き + そう) is the 'looks like / seems' visual-evidence marker — distinct from ~そうだ after a full sentence which marks hearsay. まで marks the endpoint of a continuing state ('up to 120 years old'). A lighthearted observation, probably teasing someone for being unusually healthy or lively.