Time expression + koto desu
I think that this is kinda to enphasize the time expression, but I am not sure.
Could it be used after any time expression?

Suzuki-san wa kyounen hatachi ni narimashita.
Suzuki-san wa kyounen hatachi ni narimashita koto desu.

I do not know if I could use the sentence above with "koto desu".

Question

Time expression + koto desu
I think that this is kinda to enphasize the time expression, but I am not sure.
Could it be used after any time expression?

Suzuki-san wa kyounen hatachi ni narimashita.
Suzuki-san wa kyounen hatachi ni narimashita koto desu.

I do not know if I could use the sentence above with "koto desu".

Answer

kyounen???? Don't you mean "kyonen"?

Secondly, your second sentence is wrong. It should be:
Suzuki-san wa kyounen hatachi ni NATTA koto desu.

Non perdere l'opportunità di imparare una lingua comodamente da casa tua. Esplora la nostra selezione di insegnanti di lingue esperti e iscriviti subito alla tua prima lezione!