It does not really matter.

I assuming you are talking about investment linked, but the same concept applicable to traditional product with cash value as well.

Yes, the premium for medical card depends on entry age, but COI deduction is according to each attained age.
For example, monthly premium for medical card might be 260 for age 25, and 270 for age 26 but the COI deduction is the same at age 26, no matter what your entry age is. The ending cash value will be different as well, and probably higher for age 26 with monthly premium 270.

A simple illustration below, with no investment return and expenses.

Scenario 1   
Age 25 26 27 28
Premium 260 260 260 260
COI 100 105 110 115
cash value 160 315 465 610
   
Scenario 2   
Age 25 26 27 28
Premium 0 270 270 270
COI 0 105 110 115
cash value 0 165 325 480
The only actual loss you will incur is the higher unallocated premium (~200% of annual premium) for the company expenses and agent's commission.

Furthermore, premium for medical card/basic sum assured/all other riders usually are flat for each 5 years age band.
i.e. age 31-35 or age 36-40 will pay the same premium. As long as your current age next birthday is not a multiple of 5, you probably ended up paying the same premium.