Feeling old or you and your partner don't have Sex like you used to?

So my husband I have been together for 8 years now we have 2 beautiful children together ... the thing is we want to have Sex like all the time but then usually one of us are to tired so we tried spiciness g things up adding in date night on the weekends trying to spend more quality time together and we still always say on no honey I'm to tired or I'm to sore so I guess really my question is is why are we like this even though we want to sleep with each other