I know it's not productive but this was the kind of comment I was expecting to read, thank you.

I would like to know the oldest code libraries to be reused in 'large scale' tho.

I don't know what you want to consider 'large scale', but I've definitely come across some Fortran code with dates in the comments from the 60s or 70s. Although the only I can find right now dates to 1983 (see https://github.com/nwh/lusol/blob/master/src/lusol_util.f#L3...).

DNA?