Old growth indicates here the size of mature plants at this site: up to and in excess of 3 metres.
It may be a moot(and pointless!) point, but both its size and leaves could suggest that it may be the "bohemia" variant. This hybrid is effectively a "cross" between Japonica and Giant Knotweed: there have been rare reports(not in UK to my knowledge)of Bohemia's seeds being viable. It still behaves the same!