'Thomas GRESWOLD, born about 1595, died May 1616, will proved 11 Jun 1616 PCC in Solihull, Warwickshire, England. Must have been 21, (b. 1595) unmarried; ...'
In two lists presented on page 28, in which the author points out that his father's children are listed in order (in one case the list is 'boys in order, then girls'), Thomas is listed before John. In the second list, which excludes Richard, and in which the girls are listed in birth order instead of at the end, Thomas is listed between Alice and John. Therefore, he must have been born between Oct 1588 and Nov 1591 - or about 1589/1590. Since Richard was born about the same time (if his age on entrance to Oxford in 1607 is correct), Richard and Thomas appear to be twins. This might explain why no baptismal record has been found for either of them - as twins, their survival might have been uncertain, and they might have been baptized at home shortly after birth. This source seems to estimate Thomas' birth year based on the fact that he must have been 21 in 1616 - but presumably he could have been older than 21.