I solved this issue using css styling on textItem. I don't know why it's not working via code in SmartGWT.
code:
textItem.setCellStyle("textItemAge");
css:
.textItemAge input,.textItemAgeFocused input,.textItemAgeDisabled input,.textItemAgeDisabledHint input,.textItemAgeError input,.textItemAgeHint input
{
width: 50px !important;
}