The rule of thumb is that once a live person reaches 70 1/2 RMD must start whether they remain alive of not. From there it becomes a question of which schedule is to be followed, and can be based on the deceased's age, the beneficiary's life expectancy or a five year or lump sum pay out.
The government has a vested interest in these funds hitting the economy and being dragooned by taxes. There was earlier this year an attempt to disallow beneficiaries from taking a RMD based on their life expectancy but instead to use a ten year plan. For instance, a 20yo inheriting a $60K plan would now only have a average RMD of $1K annualized for 60 years. Some parts of the government wanted this sped up to a 10 year schedule to boost spending and taxes.
This failed to get support in the Senate.