Is it permissible for someone who is unable to complete the bowing (rukūʿ) due to knee pain, and is 23 years old, to perform the prayer with this excuse, and what is the correct and minimum limit for bowing?

For bowing (rukūʿ), there is a sufficient minimum, which is to bend down such that the hands reach the knees. The complete form involves the worshipper grasping their knees with their hands, spreading their fingers, straightening their back, keeping their head level with their back, and separating their upper arms from their sides. If the worshipper performs the obligatory minimum, their prayer is valid, and there is no harm in omitting the sunnah acts if they are unable to perform them. If they are unable to perform the obligatory minimum, they should bow as much as they are able, and what they are unable to do is waived for them.