<ーっている can be used after perfect tense> 彼女は年をとっています。She has aged. 彼女は太っています。She's gained her waight. <ーです　can be used after a noun> * 彼女は年寄りです She is an old person. * 彼女はデブです。 She is a fat person(fat as a noun -Japanese expression) 彼女は女の子です。She is a girl. 彼は背が低いです(彼は背が低い人です）。He is short. （He is a short person) ※but please be awared that this is Japanese exprssion, and the grammar is not always equal to the English one. There are still some slight different points between -ている　and one of perfect tense in English. In Japanese 太っている or 年をとっている could be used as Adjective, but also as a perfect tense. However です would be always following after a noun. Hope this makes sense for you.

The rule is simple. 年をとる = get (become) old.　年をとってる = be old. 太る = get (become) fat.　太ってる = be fat. There are quite a few Japanese verbs like this. Get used to them.

You are welcome and thank you..^^ Have a safe move. Keep in touch.

Thanks Pomme, thank you very much for your detailed explanation... I will study more examples with です and います later to understand it better..... Now, I'm on the move.... Hope you are keeping well. Keep in touch ^^

There are good answers. Just the reference for you. です is for after nouns and adjectives. 彼女は“女の子”です。　...女の子(girl) is noun. 彼は“若い（わかい）”です。　...若い(young) is i-adjective. 彼は“元気（げんき）”です。　...元気(well) is na-adjective. 彼女は“背が低い”です。　...“背が低い” is small phrase. And be taken as noun. ます is for after verbs. 彼は歩き（あるき）・ます。　...歩き（＜歩く, walk） is verb 彼女は年をとり・ます。　...とり（＜とる, get） is verb. 彼女は太り・ます。　...太り（＜太る, get fat） is verb. います is a variation of ます and represent the advancing movement or the state, I think this corresponds to progressive form in English basically, but there exists some difference. 彼は歩いています。　...He is walking. <advancing movement> 彼女は年をとっています。　...She is old. <state> 彼女は太っています。　...She is fat. <state>