Meaning of the word "sons" in "Our God, Our Help in Ages Past"?

This article informs us that Watt's song was based on Psalm 90:

https://blog.reformedjournal.com/2015/01/03/time-like-an-ever-rolling-stream/

The original had three more verses that are seldom sung today, for nine in all. By aligning the verses with the Psalm, one can infer the meaning.

Here is the song in its entirety:

O God, our help in ages past,
Our hope for years to come,
Our shelter from the stormy blast,
And our eternal home.

Under the shadow of Thy throne
Still may we dwell secure;
Sufficient is Thine arm alone,
And our defense is sure.

Before the hills in order stood,
Or earth received her frame,
From everlasting Thou art God,
To endless years the same.

Thy Word commands our flesh to dust,
Return, ye sons of men:
All nations rose from earth at first,
And turn to earth again.

A thousand ages in Thy sight
Are like an evening gone;
Short as the watch that ends the night
Before the rising sun.

The busy tribes of flesh and blood,
With all their lives and cares,
Are carried downwards by the flood,
And lost in following years.

Time, like an ever rolling stream,
Bears all its sons away;
They fly, forgotten, as a dream
Dies at the opening day.

Like flowery fields the nations stand
Pleased with the morning light;
The flowers beneath the mower’s hand
Lie withering ere ‘tis night.

Our God, our help in ages past,
Our hope for years to come,
Be Thou our guard while troubles last,
And our eternal home.

Here is Psalm 90:

1 Lord, you have been our dwelling place
    throughout all generations.
2 Before the mountains were born
    or you brought forth the whole world,
    from everlasting to everlasting you are God.

3 You turn people back to dust,
    saying, “Return to dust, you mortals.”
4 A thousand years in your sight
    are like a day that has just gone by,
    or like a watch in the night.
5 Yet you sweep people away in the sleep of death—
    they are like the new grass of the morning:
6 In the morning it springs up new,
    but by evening it is dry and withered.

7 We are consumed by your anger
    and terrified by your indignation.
8 You have set our iniquities before you,
    our secret sins in the light of your presence.
9 All our days pass away under your wrath;
    we finish our years with a moan.
10 Our days may come to seventy years,
    or eighty, if our strength endures;
yet the best of them are but trouble and sorrow,
    for they quickly pass, and we fly away.
11 If only we knew the power of your anger!
    Your wrath is as great as the fear that is your due.
12 Teach us to number our days,
    that we may gain a heart of wisdom.

13 Relent, Lord! How long will it be?
    Have compassion on your servants.
14 Satisfy us in the morning with your unfailing love,
    that we may sing for joy and be glad all our days.
15 Make us glad for as many days as you have afflicted us,
    for as many years as we have seen trouble.
16 May your deeds be shown to your servants,
    your splendor to their children.

17 May the favor of the Lord our God rest on us;
    establish the work of our hands for us—
    yes, establish the work of our hands.

Psalm 90:4 has "a thousand years in your sight", which matches the fifth stanza.

Pslam 90:5-6 are the verses that match the eighth stanza, which follows the stanza in your question, the seventh.

This squeeze play shows that the sentiments in the hymn are either not in the same order as the Psalm, or must be found in verse 5. The author of the article said that Watts liked to stick close to the text of the Psalm, so that means the portion of the Psalm matching your stanza could be in verse 5 or anywhere, but should be identifiable.

The words that comes closest to evoking a stream and a dream are "Yet you sweep people away in the sleep of death" of verse 5, where sleep matches dream and sweep conveys a flood. Thus I would say that "sons" means people in general. However, the poetic meaning is that time is a Father who has sons - the hours, minutes and seconds of history. Those divisions of time come and go, their passing experienced as the dying of each day.