Joseph is found in the 1860 census as a 16 year old, born in Massachusetts, living in a State Reform School. His name does not appear in the 1850 Federal Census.

In the 1850 Federal Census, of the 10 DeCastro names appearing in Massachusetts, those not born in the state were born in Spain.

I found nothing in an immigration search

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Joseph H. DeCastro

Residence Boston MA; an 18 year-old Waiter.
Enlisted on 7/12/1861 as a Private.
On 8/28/1861 he mustered into "F" Co. MA 19th Infantry
He Re-enlisted on 12/20/1863
He was Mustered Out on 6/30/1865 at Munson's Hill, VA
Promotions:
* Corpl
* Sergt
Intra Regimental Company Transfers:
* from company F to company I
Other Information:
born 11/14/1844 in Boston, MA
died 5/8/1892 in New York City, NY
Buried: Fairmount Cemetery, Newark, NJ
Medal of Honor Information:
He was awarded the Medal of Honor
for action on 7/3/1863 at Gettysburg, PA.
(Captured flag of 19th VA Infantry)

Sources used by Historical Data Systems, Inc.:

- Massachusetts Soldiers, Sailors and Marines in the Civil War
- Deeds of Valor. How our Soldier-heroes won the Medal of Honor
- Medal of Honor Recipients 1863-1994

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1860 United States Federal Census about Joseph Decastro
Name: Joseph Decastro
Age in 1860: 16 State Reform School
Birth Year: abt 1844
Birthplace: Massachusetts
Home in 1860: Westborough, Worcester, Massachusetts
Gender: Male
Post Office: Millbury