Greetings everyone from Denmark!
Since I am not familiar at all with Dutch genealogy perhaps there is a kind soul to tell me how to find this person:
"Jan Kosterman", geb. 23.2.1844 in Holland. Immigrated via Rotterdam to the USA via the ship "Leerdam" May/June 1888. He was called "John" in the USA, and died 16.6.1910, Waukesha, Wisconsin. Buried Menomonee Falls, Waukesha County, Wisconsin, USA.
Does anyone know ANYTHING about his origins, please.
With thanks and kind regards,
Flemming Davidsen, Denmark
Do you have any information on his birthplace or the name of his wife? And how sure is the date of birth?
The name Jan is unfortunately one of the most common names in the Netherlands.
And therefore there are a lot of "Jan Kosterman' .
If he brought a wife with him maybe it's easier to identify him. Or children....
Found his grave on findagrave. His wife's name was Anna Maria. You can now continue your search via www.wiewaswie.nl
I suppose this is the right couple although they both are exactly 1 year older than on the tombstone [!]
Bruidegom:
Jan KostermanGeboorteplaats:DrielGeboortedatum:donderdag
23 februari 1843Vader bruidegom:
Johannes KostermanMoeder bruidegom:
Elisabeth AalmansBruid:
Anna Maria de KleijnGeboorteplaats:Deursen en DennenburgGeboortedatum:
vrijdag 26 juni 1846Vader bruid:
Gerardus de KleijnMoeder bruid:
Anna Maria vanden BergGebeurtenis:HuwelijkDatum:vrijdag 9 april 1875
In the census of 1905 a daughter Maria is mentioned '24 years old' This is her birth:
Kind:
Anna Maria KostermanGeboortedatum:donderdag 2 december 1880Vader:
Jan KostermanMoeder:
Anna Maria de KleijnGebeurtenis:GeboorteDatum:donderdag 2 december 1880Documenttype:BS Geboorte
Joannes Kosterman, arbeider, zoon van Joannes Kosterman en Juditha Hermsen, huwt in Nijmegen op 13.5.1826 met Anna Maria Elisabeth Aelmans, dienstmaagd, dochter van Henricus Aelmans, tuinier, en Maria Agnes Bremmers.
Gerardus Arnoldus de Klijn, geboren in Huisseling, zoon van Gerald de Klijn en Hester Megens, huwt in Dieden, Demen en Langel op 19.12.1829 met Maria Anna van den Berg, geboren in Demen, dochter van Henrici van den Berg en Elijsabetha de Laat.
Dear "Annemarie57",
you are so completely right - and the same goes for everyone else helping out. Thank you with all my heart for your time and helpfulness. Really appreciate it.
Since some of the text is in Dutch it´s my hope I may return if there´s something I cannot read or understand(speak so and so German, but reads everything, and do understand a lot of Dutch, since many words are so close to German and Danish).
A million thanks!
- Best regards,
Flemming Davidsen, Roskilde, Denmark
Joannes Kosterman x Valburg 3/9/1784 Juditha Hermsen
Children: ~rooms katholiek Eimeren: Antonius 21/2/1785 Aleijda 2/5/1786 Joannes 25/4/1788 Toon 24/3/1790 Peter 00/4/1792 Joanna 11/3/1794 Gradus 4/4/1796 Jan 5/2/1798
gr, anna
You're welcome and don't hesitate to ask if something is unclear

Joannes Kosterman, tabaksplanter, zoon van Joannes Kosterman en Juditha Hermensen, en weduwnaar van Anna Maria Elisabeth Aelmans, overlijdt in Elst (Betuwe) op 25.11.1882 (84 jaar).
Anna Maria Elisabeth Aelmans, echtgenoot van Joannes Kosterman, en dochter van Henricus Aelmans en Maria Agnes van Bremmers, overlijdt in Elst (Betuwe) op 7.12.1869 (72 jaar).
Maria Agnes Bremmers is in Maasniel gedoopt op 2.11.1762 en ze is overleden in Roermond op 30.7.1828.
Voor Maria Agnes Bremmers zie: "Google" via: "Maria Agnes Bremmers" Aelmans!
Thank you, Thank you, Thank you!
I think I may have some questions when I have registered the names in my genealogy database, but some work lies ahead, so I´m not sure yet, what may pop up. Still very happy to be able to return to you again if necessary. 
- Flemming