Not unless you assume the question asks the ages in those units for which the multiples are integral -- minutes, seconds.
You obviously did not assume that -- years are the only units mentioned in your answer.
There's no need. You can still use Year to count in minutes, seconds or even to nano seconds. For example, 2.5 years.
Precision or looseness aside, it's nice to be consistent.
Yes, that's true. So, that "66" is pretty precision, therefore, we should precisely count their birth "time".