As a general answer to your question, it almost always possible to come up with reasonable estimated dates. Put in your estimated dates, and then explain them in the biography. You gave us a marriage record of 12 September 1890, it would be reasonable to say they were likely born before 1870. (it's good enough to start a profile)
In this case, you have access to ancestry.com, it took minimal searching to find:
Ancestry.com personal site has:
William Hamilton Aitken
BIRTH 22 JAN 1860 • Biggar,Lanark,Scotland
DEATH 30 MARCH 1929 • Los Angeles, California, United States
Nellie J Aitken
BIRTH FEB 1861 • Scotland
DEATH 12 APR 1922 • Los Angeles, California, USA
https://www.findagrave.com/memorial/53884467 (wrong birth date?)
https://www.findagrave.com/memorial/107370319/william-aitken
United States Census, 1920," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MX2P-XR6 : accessed 12 January 2018), William H Aitken, Colorado Springs, El Paso, Colorado, United States; citing ED 68, sheet 4B, line 73, family 92
"United States Census, 1910," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MK43-N73 : accessed 12 January 2018), William H Aitken, Colorado Springs Ward 3, El Paso, Colorado, United States; citing enumeration district (ED) ED 38, sheet 5A, family 112
United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MQM2-TCD : accessed 12 January 2018), William H Aitken, Precincts 24, 27-29 Goldfield town, Teller, Colorado, United States; citing enumeration district (ED) 132, sheet 24B, family 674
Source Citation for Scotland, Select Births and Baptisms, 1564-1950
Scotland, Select Births and Baptisms, 1564-1950
VIEW RECORD
Name William Hamilton Aitken
Gender Male
Birth Date 01 Apr 1861
Birth Place Biggar,Lanark,Scotland
Father James Aitken
Mother Jessie Hamilton
FHL Film Number 6035516
Household Members
Name Age
James Aitken
Jessie Hamilton
William Hamilton Aitken
Source Citation for California, Death Index, 1905-1939
Name William H Aitken
Birth Year abt 1864
Death Date 30 Mar 1929
Age at Death 65
Death Place Los Angeles, California, USA
Source Citation for California, Death Index, 1905-1939
California, Death Index, 1905-1939
Name Nellie Aitken
Birth Year abt 1870
Death Date 12 Apr 1922
Age at Death 52
Death Place Los Angeles, California, USA
Scotland, Select Births and Baptisms, 1564-1950
Name William Hamilton Aitken
Gender Male
Birth Date 22 Jan 1860
Birth Place Biggar,Lanark,Scotland
Father James Aitken
Mother Jessie Hamilton
FHL Film Number 6035516
Household Members
Name Age
William Hamilton Aitken
Jessie Hamilton
James Aitken
Source Citation for Scotland, Select Births and Baptisms, 1564-1950
Scotland, Select Births and Baptisms, 1564-1950
VIEW RECORD
Name William Hamilton Aitken
Gender Male
Birth Date 01 Apr 1861
Birth Place Biggar,Lanark,Scotland
Father James Aitken
Mother Jessie Hamilton
FHL Film Number 6035516
Household Members
Name Age
James Aitken
Jessie Hamilton
William Hamilton Aitken