LXV
Letters are not numbers. Not all letters in the alphabet represent Roman numerals though some letters do.
XXV-VI
You could use roman numerals but then you would need 6 letters: XIII-V-XI.
Today we would write it out as XLV
MMMCLDXXVlll
Not possible ! Even if you were to use Roman numerals, it would be XIV-VIII-MMXIII
5909 in Roman numerals would be (V)CMIX
First write the birthdate as a number so that, for example, a date like 12th October 1537 would be written as 12-10-1537. Then replace these three numbers with the correct Roman numerals. In this case the Roman numerals would be XII.X.MDXXXVII
Just use the letters I V X L C D and M
In today's modern notation: XVI-XI-MCMXXXVIII
You cannot write it in 5 letters, you can write it in 4. Using roman numerals it would be: IX/V/MMXI, this required 4 letters.
Even if you used roman numerals, it would still require more than 5 letters.