Time horizon and stages - strategy

I have been pretty confident with this topic until I started to do older essays (prior to 2012).

Basically, in order to say short vs long term I always looked how many years remain before retirement.

Regarding the stages (single, 2 stages, 3 stages,…) I look for the cash outflows/inflows. For example, until retirement and then until death.

I did mock 2011 question 2, and the participants are 50 years old and will retire next year.

I thought this is short term, since in one year they will be pensioned, but the answer is:

The Beckers are retiring at a young age, and do not expect expenses to change until one of them dies. Therefore, their time horizon is long-term, two-stage: (1) when they are both alive, and (2) after one of them dies.

–> so it doesn’t cares about when they will retire, but only on the absolute age??

What’s the best strategy to identify long vs short and the multistages?