Humans live for more than 100 years. First 20 years they age as normal, then for another 100 years stay at 20-40, throughout which have 10 children in average, or 1 child in 10 years, which means at any given moment they have average of 2 children under the age of 20, and thus look just like a normal young family from our world, you just don't know that this 30yo looking couple are actually 70yo and their two lovely kids had several older siblings who aren't around anymore. It is common sense that an average of 2 in 10 kids make it to become 20yo themselves, but if they made it this far it means they aren't interested in getting snuffed in the first place, so they'll make it to 120 and raise another 10 kids alright, of which 2 will survive to replace them in the population count.