From the words of the Shulhan 'Arukh it would appear that the prohibition of marrying before Lagh La'Omer (the 33rd day of the 'Omer), applies when marriage is a commandment and even when it is not. It is a commandment when a man is not married and has no children, because he has not fulfilled the Torah commandment of "Go forth and multiply" (Peru Urbu).
Even an elderly widower, however, who has children, and wishes to marry an elderly widow, is not permitted to marry before Lagh La'Omer. The Hakhamim did not make any differentiation when they established their ruling.
There is, nevertheless, an exception. A man who wishes to remarry his wife (after a divorce and assuming that he is not a Kohen and that she never remarried), is permitted to do so at any time, even before Lagh La'Omer. This is in the same way that he is permitted to remarry his wife on Hol HaMo'ed, because the Simha (happiness) in the case of a remarriage, is not at the same level of one who is marrying a woman for the first time.
(See Shulhan 'Arukh, O.H. 493:1, 546:2. Kaf Hahayyim, 493, Oth 2, 3)
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