Snapedom

There is no reason for wizards to copy Victorian customs without Victorian living conditions. Wizarding culture parted from the culture of people in the late 17th century and went its own way. If infant mortality for late 20th century wizards is low and has been for a while there is no reason for them to have early christenings. (And in any case, the story about Harry's hurried christening isn't canon. It never made it to the books, including the supplementary ones.)

There is no indication in canon that the Longbottoms were ever in hiding, or that they knew they were threatened because of Neville (rather than because of their role as Aurors and Order members). It isn't even clear if the Potters knew they were being targeted because of Harry rather than because of their own roles as order members, but at least they knew they were being targeted specifically. It is entirely possible that in 1981 the only people who knew of the prophecy were the brothers Dumbledore, Tom, Severus and the relevant Unspeakables.