When the home inspector says "the roof is fine but probably will need to be replaced in three to five years" it isn't reasonable to ask the seller to replace the roof.

When the home inspector says "the heating system is working just fine but is beyond it's statistical life expectancy" it isn't reasonable to ask the seller to replace the heating system.

But... it is reasonable to think differently about the value of a home if the roof and/or the heating system will need to be replaced soon.

So, simply put, ask about the age of the roof and heating system *before* making an offer!