Some scholars are of the view that a birthday party celebration is impermissible due to the fact that these birthday celebrations involve imitation of the innovations of the non-Muslims in their birthday celebrations.
Ibn ’Umar narrated that the Messenger of Allah (ﷺ) said:
“He who imitates any people (in their actions) is considered to be one of them.” Reported by Abu Dawud and Ibn Hibban graded it as Sahih.
Other scholars have allowed it as a cultural celebration, within guidelines, and with the participants knowing that the celebration has no religious significance. They claimed that the prohibition is clear on the religious celebration, however, holidays/celebrations that do not represent religious traditions and are not gathering with worship overtones or unIslamic activities, do not carry the ruling as the religious ones.
In summary, celebrating or attending a birthday gathering, while not required in our religion would become allowable or prohibited based on what’s done in it. There is no harm in celebrating the occasion with friends and family without the exploitation or practice of impermissible acts.
You may follow whichever opinion you are convinced with, but we should not rebuke others for what they follow when they are supported by credible scholarship.