My argument for the Queen's cousins to be retired isn't based on age, I just don't see them as necessary now that there is a third generation taking on more royal duties.
Some people are quick to point out that Beatrice and Eugenie are not necessary to the firm, party based on age and distance from the throne, so the same argument should stand for the Kents and Gloucesters too.
Personally, I like the Dutch system where membership of the royal house is restricted to relatives of the current monarch up to the second degree of consanguinity (and their respective siblings, when applicable), while the line of succession is slightly broader and goes up to the third degree.
That means in practice that the royal house includes siblings, children and grandchildren of the current monarch, while the line of succession includes all of the above plus great-grandchildren, nephews/nieces, and surviving uncles/aunts (in case there are any), but not any cousin.
If applied to the current BRF, that would mean automatic exclusion of the Kents and the Gloucesters both from the royal house and the line of succession, but would still translate into a fairly large royal house, including the Queen and the Duke of Edinburgh, plus Charles, Camilla, William, Catherine and Harry; Andrew, Beatrice and Eugenie; Edward, Sophie, James and Louise; Anne and Timothy Laurence, Peter and Autumn, Zara and Mike Tindall. Prince George, Peter's and Zara's children, and Princess Margaret's children would not be part of the royal house, but would still be in the line of succession, exactly in the same positions where they are today.
Once Charles ascended the throne, George would enter the royal house, but Anne's, Andrew's, and Edward's children would be excluded, though still remaining in the line of succession. Margaret's, Peter's and Zara's children would be out both of the royal house and the line of succession.