We have been traveling with our kids since before they were even a year old & the only place where I have had questions raised is at 2 European hotels. Some have very strict rules where they won't allow more than 3 guests in a room, no matter what the age. I remember a few years ago in Switzerland they made a fuss about my kids staying in the same room when they were 5 & 3 years old. They eventually upgraded me to a suite which allowed 4 guests but charged me for an extra bed. Then I faced a similar problem in Greece the following year. Another upgrade permitted all 4 of us to stay in the same room but they charged for an extra person.

My kids are now 10 & 8 years old & I have not faced a problem booking a single room for all 4 of us in countries like India, US, UK, Canada, Singapore, Thailand, UAE, Japan & France. The system will only allow me to book for 3 so I do that & when I turn up at the front desk I don't try to hide the fact that there are 4 of us. Never has a hotel had a problem with that besides the 2 instances I mentioned above.