I have this feeling you're unsatisfied with those answers.  Boosting a sailor will not only give it 20% more growth, but also 20% more ability. Therefore, it does not make a difference in ability for different sailors boosted at different levels. Meaning, boosting as early as possible is for your own convenience.

Example 1: Non-boosted sailor

Growth 10, Ability 30 sailor

lvl 1:Start at 30 ability

lvl 2: now 40 ability

lvl 3: now 50 ability

If you boost it at lvl 3, it will show + 12 growth and 60 ability

Example 2: Same sailor as above, boosted at level 1

Growth 12, Ability 36 sailor

lvl 1: Start with 36 ability

lvl 2: now 48 ability

lvl 3: now 60 ability

Now you see how you can boost at any time and still receive the same result, but by boosting at level 1 you had the extra ability working on your accuracy/reload/repair/overheat/whatever.