I know guys who say they're ok if they miss 2 days, but swear that if they miss 3 or more, they're hopeless. Being 38 (ie, 'old' 🙂), I find the more rest I get the better -- I'm even thinking about cutting down to 4 days a week on the bike instead of 5 or 6.
I suppose the obvious question is, has this happened to you before?
We're obviously not all the same, so it's probably mostly a matter of figuring out what your legs like. Having said that, there are days when we do everything right, but the legs feel like [censored], and there are also days when we're supposed to be knackered, but we feel fantastic. Unfotunately, this unpredictablity seems to increase with age :p
In my opinion, motivation also plays a HUGE roll. Some days there's just nothing that gives you a spark.