The belly, in Psalms 16:4, signifies the interior understanding.

In Genesis 25:23, it signifies conception of interior and exterior good. (Arcana Coelestia 3293)

In Isaiah 46:3, the belly signifies from the beginning of the church. (Arcana Coelestia 4918[2], 5550)

The belly signifies people who are being regenerated, and people who have been regenerated. (Apocalypse Explained 710[3])

(References: Apocalypse Explained 622)

5550. 'You will cause my grey hair to go down' means that this will accordingly be the final phase of the Church. This is clear from the meaning of 'grey hair', when the Church is the subject, as its final phase. A final phase is also meant by 'grey hair' in Isaiah,

Hearken to Me, O house of Jacob, and all the remnant of the house of Israel who have been carried from the womb, borne from the belly. Even to [your] old age I am the same, and even to grey hair I will carry [you]. Isaiah 46:3-4.

'The house of Jacob' stands for the external Church, 'the house of Israel' for the internal Church. 'From the womb and the belly' stands for since it began. 'To old age and to grey hair' stands for the final phase of it. And in David,

Planted in the house of Jehovah, they will flourish in the courts of our God. They will still have produce when in grey hair. Psalms 92:13-14.

'When in grey hair' stands for when in the final phase.