I would say it is opposite. It would be more red flag for ml to be childish. He is mature character ho acted mature. He literally put distance between him and mc after the act. Mc is the one who push him to the corner. Like ml is humantoo. They were 'both caught up' in the act. Usually, its pretty normal more mature person to guide other one in relationship. This isnt red flag. It would be red flag, if it involved manipulation or grooming or sexual harassment. Like, its not mls fault that mc admired him so much. There are a lot of relationship where people end up with their childhood crush or maybe celebrity crush. Yesz He is experienced actor and in a sense, senior but they hardly had relationship before as 'senior-'. Ml isnt mcs dad or teacher or someone who will always above him'. With that logic, all age gap relationships would be red flag. 'He shouldve rejected him', why? They are both adults. 'Mc is confused' so is ml. And mc literally lash out to ml and people around him for not taking his love seriously. He is not a child. They are both just actors in the end. And ml doesnt have to treat him like baby. Ironically, this is one of the stories where age gap is done right and realistic.
I can ignore age gap when it exists only in the name. Where the characters act so out of character for their given character background they're given that the age gap and the resulting power gap doesn't manifest at all. But, not here. The characters are realistically depicted. But, that also means that the age gap and the power gap becomes real.
The older actor is significantly more mature than the younger one. He is also significantly more accomplished. He is clearly a mentor figure to a younger person who looks up to him and significantly more child-like, in a very real sense, compared to himself. While 24 and 31 are not far apart enough to raise eyebrows in every situation, this is definitely a situation where it would be concerning. Because the effective age gap is a lot larger because of additional power gap and hierarchial gap.
It would be really gross for the older actor to encourage and misuse his young colleague confusing respect and gratitude as sexual interest. The responsible and the moral thing for the older, wiser person to do is to stop the boy's confusion as soon as he realises it and help him see wisdom and to not soil the mentor-student relationship. This kind of a character dynamic would be better served in a shounen. The older character is giving a much older sibling vibes. Romance stories should not be written around these kinds of dynamics. A good shorthand for what kind of dynamics should not be sexualized is if at any moment you feel that an inter-character dynamic can be likened to a familial dynamic in anyway, that's your sign to stop writing a romance centred on these characters and go back to the drawing board. Acquaintances energy. Green. Friends energy. Green. Family energy. Big flashing Red flag.