the short answer:

any transaction set coin-age of the involved coin pile to to zero

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the long and confusing answer:

u have to think of your wallet as a tresor with a lot of little pile of coins inside

when u receive a transaction than a new little pile is put into ur tresor

this little pile have a coin-age of zero

when u send away coins than ur tresor take the youngest pile and remove the amount of coins u want to send

BUT the coin-age is set to zero for all coins in that little pile

thats the reason why people developed coin control so u can select from which little pilet u want to use the coins

the basic logic isnt optimal by automatic selecting the youngest pile because this could also be a pile with 10000 coins and age 7 days

but thats still lot more coin-age than a pile with 10 dmd age 30 days

ok basical i overanswered ur question sorry

our POS code try to counter the risc of losing coin-age by split big coin pile into smaller part when u successfull mint between 7-30 days coin-age

if u mint above 30 days coinage pile is not split but instead if u have multiple small pile they group together (up to 100 DMD) and increase the chance to recieve a POS reward

in one of the next wallet releases we will also add coin control maybe even in a advanced version (what other coins have now as coin control is very basic and required the user to choose the pile he wana use)