Basically, if population is getting older, and new investments (software) do not generate much employment and hence multiplier effects (and indeed induce people to work less, because what is better than checking your Facebook page all day long?) then GDP growth will slow.
Hansen argued that an economy needed strong and healthy levels of investment if it was to maintain full employment. He wrote: “For it is an indisputable fact that the prevailing economic system has never been able to reach reasonably full employment or the attainment of its currently realizable real income without making large investment expenditures.”
via CONVERSABLE ECONOMIST: Secular Stagnation: Back to Alvin Hansen.