The point here is that an "ever since" state situation is being described which uses the present tense with depuis que in French (as locution conjunction) https://www.wordreference.com/fren/depuis
but it does translate as past tense in English, I suspect that this is the reason for your query?
There is no continuous present in French -- the present is made to suffice with respect to your query about savoir. The past perfect usage, later on, confirms the completed action (you told us)
Hope this helps?
Le Présent is used with expressions such as "être petit(e)" or "avoir ... ans" because it implies a permanent/ongoing state rather than a one time completed action.
Depuis que j'ai douze ans, j'aime les romans policiers = Ever since I was twelve, I have loved crime novels
Je connais Marc depuis qu'il a trois ans = I have known Marc since he was 3 years old
Le Passé Composé is used if what directly follows "depuis que" describe a completed one time action:
-> the twelve birthday was only celebrated once - therefore it is a completed one time action
To supplement Jim's answer follow the links below:
why not the present tense
can we say depuis que je te connais
can the verb following depuis que be in the present tense
I hope this is helpful.
Bonne journée !
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