Should I worry about this? (does it reduce the effectiveness of the shingle). If yes, then question 2 come into play.
There's certainly more important things to worry about in life, but, yes, Moss will, over time, shorten the lifespan of asphalt shingles.
Can it be corrected in the area that are truly unsightly with a chemical or something? I have kids so it would need to be safe. If yes, question 3 comes into play.
Yes. Actually, you want to correct it over the entirety of the roof. You can purchase spray-on as well as granule based moss killer. That is what you want.
If some cleaning can be done to fix, is this a DIY or does a service have to be contacted?
You can DIY or you can hire a service. If you live in a moss-friendly region (such as the Pacific North West of the USA) then you'll find all sorts of people that do pretty much just this. Either way, be sure to hire people that use very gentle cleaning methods. You don't want power washing or anything of that sort.
Once corrected, is there any preventative measures that can be done to avoid this?
The only real remedy is to not shingle a roof with asphalt shingles in moss-prone regions. That's why you see a lot of standing seam metal roofing in wet areas. Barring that, you just need to make the moss prevention an annual chore.
Some claim that zinc strips will work (as zinc will kill the moss) though, at least around here, you can find plenty of roofs with zinc strips that also still have plenty of moss on them, so I tend to think that's a bit of a scam solution.
It depends on what your goal is. If the goal is to never have water enter your house in any way then probably what you need to do is get one of the shingles that fell off and see what its lifetime rating is. Most are 15-20 years. If you are near that point and shingles are coming off then I would think about it.
However shingles falling off doesn't necessarily mean you need a new roof. It could have been a bad install on that part of the roof - or everything.
What you read is pretty true though except for the vegetation part. If you have tree coverage over your house you might get moss with a 2 year old roof. The curled, cracked, badly warped shingles... this is extreme and should be replaced.
If you want to wait to the last second, then you need to venture to your attic a lot - after big rains. Most of the time a roof will show signs of moisture before it starts letting in a lot of water - unless your roof is pretty flat. If you don't want to get up in the attic to do this then it is purely a guess/luck.
Last thing... How long do you plan on being in the house? If it is less than 10 years. Do you plan on replacing before reselling? If yes and yes. Then just replace it and enjoy the roof.
Best Answer
Assuming a shingle roof. When to replace:
The climate affects the expected life--heat is hard on shingle roofs. Are you comparing the lifespan to other shingle roofs in your area? That is a better indication than opinions from people around the world.