It really depends on the story, how is the world and the rules in it, what you want your MC to experience to later define him and what you want the readers to know first on about the characters.
For example in https://royalroadl.com/fiction/1439/forgotten-conqueror
the MC starts with his memories and power limited only by his infant body, but one of the most emotional moments is when he's 1 year old. His family and a lot of the basics of the world are introduced then and his goals to find out why and what is he doing here and now while his past life is left vague. After that come time skips.
the MC becomes aware of himself a bit later skipping the baby phase and finding himself in a hell hole for a very long time, hating his new life, this new wolrd and the reincarnation itself to the point of serious mental and health issues and barely escaping death
In Youjo Senki, the MC has just a few flashes of consciousness while growing up, trying to figure where, why and what is going on, piecing past memories and reasons in total lack of control of bodily and mental capacities and in "several moments" years pass.
There is also one more here on rrl that i've been meaning to read about a child in a royal court with intrigues and conspiracies. As I understand it that won't be the only plot line, but it's slow paced and that time frame is gonna be a long arc in the story.
So it all depends on how and what you want to present it and what you are most comfortable with, it's up to you. Be it short, long, skip it altogether, flashbacks or just op child.