It had originally been the hope that the pair would be in the United Kingdom for the start of the English season, but both are still involved in cricket in their homelands.
New Zealand batsman Young is involved in the Kiwis' home One Day International series against the Netherlands, that doesn't end until next Tuesday.
While Kelly is part of the Western Australia team that has reached the final of the five-day Sheffield Shield that started on Thursday morning. They are taking on Victoria at the WACA in Perth.
It means that Young and Kelly are likely to make their Northamptonshire debuts in the home date with Yorkshire on Thursday, April 21, with Northants not involved in the second round of Championship matches that start on April 14.
The County are due back in pre-season friendly action today (Thursday) when they host Durham (start 11am).
It is scheduled to be a two-day friendly, and will be played behind closed doors.