This is the birth order of the Ryan children.
Frank
Kathleen
Pat
Mary
Siobhan
Somewhere during the time of Rae Woodward it was revealed Frank was born in late May 1946, making him 29 in 1975 when the show began.
I am pretty sure in the first year on air 1975 it was mentioned Mary was 22.
So knowing Siobhan was the baby of the Ryan kids and if Mary was born in 1953, it is possible Siobhan could have been born in 1954, 1955 or 1956. Thus making her age 19-21 in 1975 and able to have graduated high school already.
But I should also add that somewhere during the shows run it was revealed Maeve had 2 miscarriages so since Johnny and Maeve had no high school age kids when the show started in 1975. Maeve must have stayed pregnant most of late 1945 through 1956, YIKES!!!
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