Jean - only the 1841 census asked for ages to be rounded - all later ones asked age (usually interpreted as at last birthday tho this was only explicitly asked 1881 + 91)
John s/o Wm had a baptismal date 23rd Feb - the 3 censuses were 30/31 Mar 7/8 Apr and 2/3 Apr so only the 71 is out but only by a year

The only other Manx born John Taggart in your range was born 1800 s/o Matthias Taggart with mother Dinah - that rare name I would expect to see repeated in grandchildren

But as you have also found the name Taggart is quite common in NW England + also Ireland