I was four years old at the time.
( I ( ( was ( ( four years ) ( old ( at ( the time ) ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (ADJP (NP (CD four) (NNS years)) (JJ old) (PP (IN at) (NP (DT the) (NN time))))) (. .)))
I was four years old in 1980. 
( I ( ( was ( ( four years ) ( old ( in 1980 ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (ADJP (NP (CD four) (NNS years)) (JJ old) (PP (IN in) (NP (CD 1980))))) (. .)))