There is certainly nothing wrong with being friendly with students; all other things being equal, it is desirable to do so. However, it is important to ensure the being friendly does not detract from your authority and objectivity as a teacher/assessor of their work.
In regards to reviewing marking of assessment, you should make it clear to students that you are open to considering questions/objections on your marking, but once those questions/objections are made, you will make a professional judgment and decide on an appropriate response (including the possibility that there is no change in marks). Frank objections to your marking are fine, but frankness is a two-way street --- you are the subject-matter expert here, not them. In terms of process, you should correct your marking if you genuinely believe you did it wrongly (i.e., if the student gives a convincing explanation of a problem with the mark), but you should not allow yourself to be brow-beaten into raising marks simply because a student is upset, or unreasonably persistent. Listen to their explanation, make a professional judgment on its merit, and respond in a calm but firm manner. If the student seeks to persistently argue the point ---beyond what is reasonable--- you can calmly state that you are not convinced by their explanation and you do not propose to change the marks you have allocated. In the rare case where a student escalates to the point of rudeness, you should try to remain calm and friendly, but let them know they need to take it down a notch.
As a secondary point, take note of the laws of economics: if you incentivise students being horrible to you (by rewarding this with unwarranted increases in marks) then they will respond to this incentive by increasing the supply of horribleness. If you allow students to argue with you in a belligerent manner, and do nothing to draw attention to this, they will hold steady the supply of belligerence.
Now, since you're only twenty-five, you're not that much older than the students you are teaching. This is inherently going to mean that you have more in common with them, you may be seen as more approachable/soft, and you are more likely to be seen as a friendly figure with less authority. (And indeed, since you are junior, you probably do have less authority than the senior professors.) That comes with benefits and drawbacks. I recommend you try to maintain your instinct for friendliness, but ensure that you maintain professional objectivity and authority.