According to the timeline established in the Dune Encyclopedia (see below), Stilgar was born in the year 10,141 A.G. We know from the original Dune novels that Leto I died in the year 10,191 A.G. and that the God Emperor Leto II was born in the year 10,209 A.G.
This means that Stilgar would have been 50 during the events of Dune, 68 at the birth of Leto II (as depicted in Dune Messiah) and 78 when he ascended to the throne, as seen in Children of Dune.
BEN FIFRAWI (10,141-10,228). Most generally described as Stilgar the Fremen or Stilgar the Naib, but in his youth as a wali in Umbu Sietch, known by his birth-name of Tuan. It was not until 10153, when he became a sandrider and accompanied a group of other youths on a raid against a Harkonnen village that he acquired the name by which he would be best known. (His troop name, used only by his comrades at Umbu Sietch, is thought to have been Sahkan—the Fremen name for a type of desert hawk—but verification of this point is difficult to obtain. The evidence found thus far consists of a reference made by a man from that sietch who accompanied Stilgar on jihad.)
The Dune Encyclopedia
and
LETO ATREIDES (10,140-10,191) A distaff cousin of the Corrinos, he is frequently referred to as the Red Duke.
Dune: Appendix IV
and
10140 - LETO ATREIDES (Duke Leto I) born.
10154 - LADY JESSICA born.
10155-10165 GURNEY HALLECK a Harkonnen slave on Giedi Prime.
10158 - DUNCAN IDAHO (the human) born.
10175 - PAUL ATREIDES born.
10191 - The Sardaukar attack on Arrakis kills Duke Leto; Paul and Jessica go into the desert with the Fremen. ALIA ATREIDES born.
10196 - SHADDAM IV abdicates; Paul becomes Emperor.
10200 - FARAD'N (KENOLA) born to Wensicia and Count Dalak Kenola.
10209 - LETO II and GHANIMA born. CHANI dies. Paul goes into the desert, and Alia becomes regent.
10219 - LETO II ascends to the throne.
The Dune Encyclopedia