I'm 27. i've been prone masturbating since my teenage and i find no priblems in erection. is it safe to masturbate prone ?

The biggest issue: Usually revolves around the habits that you develop around frequent masturbation. It can lead to habits which make intercourse with a significant other very difficult to your body becoming accustomed to particular forms of stimulation. Also, if and when you are an a serious relationship your significant other may not be as receptive to any continued frequent masturbation.

Sensitivity changes: Frequent male prone masturbation places significant pressure on penis & nerves at its base & may cause numbing to arousal / pleasure with other positions. Seek help for delayed orgasms, erectile dysfunction or inability to achieve orgasm in other ways. It may lead to back pain in males & females. REF: https://www.healthline.com/health/healthy-sex/prone-masturbation#effect-on-sensation