I think I don't know of a more user-friendly software than YouTube.
I have one cousin that learned how to search on it since 3 and a half years old and another at 4 years old. The 4 year old learned to search on their own by hitting the mic icon and saying what they wanted to watch.
And you don't even need to know how to search to use it. It's just a matter of selecting a video from the recommended list and hitting the "skip" button. That's all.
I wouldn't be surprised if a baby learned how to use it from the moment they're able to see clearly and have enough motor skills on their arm to tap stuff on a phone, maybe after watching their parents select videos for a few days.
I'd be hard-pressed to find a 5-year-old that wouldn't already know how to use YouTube.
It's probably easier to train a dog to use YouTube than to play fetch. (maybe this last one is an exaggeration... maybe..)