Naturally, the National Records of Scotland Marriage registration has the date of the Marriage, as well as the age of the participants (the bride and groom). From this it should be relatively easy to deduce the approximate year of birth. e.g. A 20 year old marrying in 1912 would have been born in 1891, if their birthday that year was after the marriage date, or 1892, if their birthday that year was before the marriage date.

The structure of the register document looks a bit like this:

1912, MARRIAGES in the District of Dornoch in the County of Sutherland

No. When, Where and How Married Signature of Parties.
Rank or Profession, Whether Single or Widowed, and Relationship (if any)		 Age Usual Residence Name, Surname and Rank or Profession of Father.
Name and Maiden Surname of Mother		 Signature of officiating Minister. Signature of Witnesses When and Where Registered and Signature of Registrar
1 1912
on the Sixteenth
day of January
at Dornoch
after Bannes according to the forms of the Free Church of Scotland		 (Signed) Kenneth Fraser
Firsherman
(Bachelor)		 31 Embo, Dornoch Peter Fraser
Fisherman

Christina Fraser
M.S. Mackay		 (Signed) E. MacLeod, Free Church Minister, Dornoch
(Signed) William Mackay. Witness.
Johan Fraser. Witness.		 1912
January
16th
at Dornoch.
J. Sutherland

Registrar
(Signed) Jessie Mackay her X mark
William Mackay Witness
Julia Mackay Witness
(spinster)		 28 Embo, Dornoch Alexander Mackay
Fisherman
(deceased)

Christina Mackay
M.S. Grant

Since the registration entry has the age of bride and groom, it should be easy to deduce an approximate birth date. However, a few times it has been incorrect, or completely illegible.

In the case where the age is wrong or illegible, it is necessary to take a wider approach to finding a corresponding birth registration. To that end, knowing at what year to start the search is important.

The NRS Scotland website says this:

What Was and Is The Minimum Age For Marriage in Scotland?

Before 1929, Scots law followed Roman law in allowing a girl to marry at twelve years of age and a boy at fourteen, without any requirement for parental consent. However, according to one early 20th-century source*, marriage in Scotland at such young ages was in practice almost unknown. No doubt if marriages between children had become common, there would have been public pressure to raise the legal minimum age of marriage earlier than 1929. The Age of Marriage Act 1929 (applying in Scotland, England & Wales but not in Northern Ireland) made void any marriage between persons either of whom was under the age of sixteen. Sixteen remains the lower age-limit today, contained in the current legislation, the Marriage (Scotland) Act 1977. Scots law still has no requirement for parental consent.

*Source: Vital registration: a manual of the law and practice concerning the registration of births, deaths and marriages. (G T Bisset-Smith. 1st edition. Edinburgh: William Green & Sons, 1902)

See: What Was and Is The Minimum Age For Marriage in Scotland?

So, for a girl marrying in 1912, birth searches should be up to and including 1900. For a boy they should be up to and including 1898. However, as the excerpt above says it was not common, and realistically it is unlikely that a person would get married before their 16th birthday.