「そうか。やはり私のほうが年を食ってる。私は今年で２０うんたら才だ」「いや、わかんねーよ」

「。。」「、」

'I see. I'm the older one after all. I'm twenty-whatever this year.' 'Wait, that doesn't tell me anything.'

Literal

''I-see. As-I-thought I [possessive-の] direction [subject-が] year [object-を] eating-[state-てる]. I [topic-は] this-year [locative-で] 20-blah-blah years-old is'' ''No, not-understand [emphatic-よ]''

A playful evasion of revealing one's exact age. 年を食う ('to eat years') is a rougher, more self-deprecating idiom for aging than 年をとる ('to take on years'). 2〇うんたら才 ('twenty-something, twenty-whatever') uses うんたら as a non-committal placeholder for 'blah' — a casual filler when the speaker doesn't want to commit to a precise number. わかんねー is rough/masculine わからない ('don't know').