Pvt. Mark O'Brien, 36, center, from Portsmouth, N.H., and Pfc. William MacGillivray, 37, an arborist from Arlington, Mass., left, sit with younger recruits as they take part in basic training at Fort Sill Army Base, in Oklahoma. While the number of older recruits, more than 3,800, is small by Army standards, the pace of over-35 enlistment jumped sharply in the first months of this year. Motives vary from a yearning for midlife adventure to a desire to serve their country. But rising unemployment is also a major reason, say Army officials, recruiters and training officers.