Can you substitute "laj" for "lay" in Creole?

No.  They each mean something different.

Laj (n.) → age (n.)

example:

Ki laj ou?

How old are you?

Ki laj li?

How old is he/she?

Ki laj papa w?

How old is your dad?

Ki laj kay ou a?

How old is your house?

Mwen pap janm di ou laj mwen.

I will never tell you my age.

Lay → garlic

example:

Mete yon ti lay nan manje a tanpri.

Add a little garlic in the food please.

Lay bon pou sante ou.

Garlic is good for your health.