Clownfish aren't sad/stressed or anything when they don't have an anemone to hang out in. That's a human emotion and the fish just "react" and that's it.
The 6-month mark is pushing it but let's back up just a moment... it's not the Anniversay Date that you're looking for. That's just a random number literally pulled out of thin air. The only real connection to 1-year and a tank being ready is the fact that many times if someone stays in the hobby a full year they've already made most of the beginner mistakes and usually have a stable system. With that being said some people can have a very stable tank in just a month or so if they used existing live rock, live sand and know what they are doing. Then some people who have been in the hobby 2 years literally are still at the starting gate because they haven't found their own way and have done nothing but try this and try that and never really learned why they are doing anything.
When I help someone set up a new tank I tell them this (and most want a Clownfish and an Anemone just like the movie).... when your tank is STABLE and HEALTHY for 9+ months then you can start looking for an anemone or at least start researching and making sure you have the proper equipment to support one long-term (lights, pumps covered, etc). Just because the tank has been "wet" for 12 months means absolutely nothing. If the tank is 11months and 29 days wet but the owner has had on-going problems over the last few months at all... the tank is not ready for an anemone.
We all love the idea of the symbiotic relationship between the fish and anemone but unfortunately many of us aren't ready for that and some of us never get to that point.
Also as Kongor pointed out sometimes they aren't a good idea especially if you have other coral in the tank. The anemone will ALWAYS go where it gets the right light, flow, and everything else and that's usually behind a rock or right in the middle of your expensive coral garden. Sometimes they can wander throughout the tank wiping out coral after coral for no apparent reason. I've seen them start moving after being stationary for months LOL!
Re:FrogSpawn as a host - I have indeed lost several very nice colonies of FrogSpawn and hammer because my clown "loved them to death". Fact of the matter is these coral have very fragile flesh which is in close proximity to hard and sharp skeleton. This can be harmful with too much current but is often the detriment to the coral when a clownfish is "rough loving" on them. Once their flesh is damaged you have a high probability to get Brown Jelly Disease which can wipe out an entire tank of Euphyllia in a matter of just a couple of days. One head/colony is damaged and the BJD spreads like wild fire seemingly over night. Something to consider.