Like I said some people prefer to set rules that are consistent for all and just have that the case from the beginning - that clearly works for you and great. Why not. Nothing wrong with that.
In case you are being genuinely curious:
I watch and spot for falls, although really a fall from a sofa is unlikely to cause a huge amount of harm to a toddler, so I would not be terribly concerned about it being a danger (I'm referring to my own children - likely the tolerance for danger would be different if I were looking after other people's). The tall part of all our sofas are currently against walls anyway, so you could only fall from the seat height. When they were not, we would always have an adult nearby if a baby wanted to climb on the back of them or take them down and engage them in something else if we could not watch.
No set age - probably around 5-6, maybe depending on the exuberance of the play - this would be a dialogue and case by case basis from probably 2-3 years. For other people's sofas, I'd likely have hold of a child up to approx 3, although IME they are generally a bit reserved and not wanting to fully explore new places once they are past 18-24 months anyway, so it wouldn't come up, except if it's a childcare setting that they're meant to treat as home. Older than that I would tell them not to climb, but generally by that age, they are copying the example of other kids in the house so if they aren't climbing then they won't either (and if they are then it's probably fine with the parents).
I don't mind if multiple children climb on my sofa. I would purposefully not buy new sofas while we have small children. But if I did I would probably select for robustness, maybe cheapness as well. Certainly if I was a childminder, I'd be looking for washable covers, possibly replaceable parts (IKEA?) and cushions that don't misshape. For me, things in my house suffering wear and tear is par for the course and not worth a constant battle and stress to keep things pristine when DC are little. I would rather accept that it's likely to happen and work around it. So we have mostly IKEA, cheap, easily fixable, wipe clean furniture. Some people would find that really difficult/depressing, so perhaps it's more of a priority to them to teach children to respect furniture at a younger age. I quite like IKEA stuff so I don't mind at all.
I just explain to my children at 3/4/5/6/etc why something is OK or not and they generally are fine or we work out some kind of compromise or different activity - they seem to be able to understand just fine that babies don't know everything yet or are smaller or aren't deliberately doing something or don't mean to hurt when they hit out of excitement. If it's causing a real issue (I remember once the baby was sitting on a box or toy or something that when the 3yo sat on it, it deformed but he was upset that the baby was allowed when we took him off) then I would stop letting the baby do it as well or move everyone onto a different activity. I don't tend to enforce stuff by punishment so maybe that's why it doesn't feel unfair? It's not like hey, when the baby does this you let them but when I do this I get put in time out. Because that's not what's happening. It's just a kind of constant assessment and figuring out what's working for each person right now. It's a bit the same as when an older child is allowed to do something (maybe play in the garden alone, play a computer game, use some complicated or messy art equipment, drink fizzy drinks/eat boiled sweets etc) but a younger one is not. You can distract them or work through their disappointment just the same as in that situation.
As I said - not a childminder, so only based on my own children but having looked up the ratios I think this approach (combined with a pretty robust childproof area for childminding in and being confident that things like safety gates are fastened, not being too worried about damage to furniture etc) would be totally managable with up to 3 under 5s plus up to 3 older children.