how do you tell marking? 4 month old is too young usually starts when they figure out they can lift their leg about 6-9 months
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You will find dribbles on every doorway, furniture legs or any post like structure in the house.
If you are planning on neutering do it no later than 6 months to prevent this, if not plan on buying the belly bands, line it with lady panty liners (which needs to be changed often) before they start!